Digital Field Mixer

Messiah College Engineering

Senior Project 2001 - 2002

Nathan Charles Jonathan Norton Jonathan Owen George Sundin

0.1 Abstract Much of our reality is shaped by sound that we like to preserve. There is

a large gap between high end professional and consumer equipment. We have produced a digital field mixer with professional expandability that will help to bridge the gap between these worlds. This group consists of Nate Charles Jonathan Norton, Jonathan Owen and George Sundin. Our advisor is Dr. David Gray. 0.2 TOC

1 Intro

1.1 Objectives 1.2 Literature review

2 Design Work 2.1 Experimental Work 2.2 Design Segments 2.2.1 Analog 2.2.2 Digital 2.2.3 Other 2.3 Twist and Turns

3. Implementation 3.1 Construction 3.1.1 Analog 3.1.2 Digital 3.1.3 Other 3.1.4 Physical 3.2 Operation

4. Schedule 4.1 Gantt Chart 4.2 Explanation 5. Budget

5.1 Prototype vs. Projected Production costs 6. Conclusions 6.1 Objectives Met 6.2 What We Learned 7. Recommendations for future work

0.3 Acknowledgments We would like to thank the following. God – the Limited Sanity for which remains. Steve Dove - Example Code, Layouts, and Reference Materials. Dr. David Gray – Direction and guidance in our project and life.

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Jon Meyer – Shop assistance Earl Swope – Valium

Dr. Pratt – Senior class Advisor Kathleen Skinner – Group mom and loving (and understanding) fiancé to

Jonathan Norton

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1. Intro 1.1 Objectives

We will design a small audio mixer that has the following features: • 4 microphone/line level inputs with a signal to noise ratio of 90 dB • A 3 band equalizer on each channel allowing ± 15 dB of gain at the chosen

frequencies • A digital output adhering to the S/PDIF specification of the main

Left/Right output channels • An expansion bus adhering to the I2S specification that allows the ability

for additional devices to be attached directly to the DSP. • Battery operation for at least 2 hours

1.2 Literature review

There are several categories in which our project fits. The first is the high-end stand-alone field recorder. These devices have built in mixers and record to DAT, Mini Disc, Flash, Hard Disk or DASH. The indisputable king of these devices is the Nagra D-II. This device has phantom powered microphone pre-amps, digital inputs and outputs and records to DASH, a digital reel-to-reel format. These features are packed into a unit that is one of the most rugged in the industry. Nothing good comes without its cost, however, the Nagra D-II retails for $17,000.

The other category that our mixer fits into is the field mixer. Field mixers see use on movie sets, in reporting settings and in all manner of live arenas. Current models are analog based with one exception, the Aeta Mix2000 that has an optional digital output card. This field mixer has 4 microphone inputs with phantom power, 2 outputs and an option for digital output and retails for $3695. These are two of the best units in the market and our design offers many of the features of the Nagra while offering more than the Aeta.

A primary component in our mixer will be the Digital Signal Processor, or DSP. DSP technology has come a long way from its early days. Originally conceived as a method of simulating the performance of complex analog devices DSP started life as just a set of algorithms. As people realized the potential of DSP, they began to strive towards the goal of being able to perform DSP in real time. This goal was ultimately achieved and now DSP has reached the point where it is often used in place of the analog components it once simulated. There are many reasons for this, the most important of which are cost and expandability. First, DSP offers a cheaper alternative to analog. As the complexity of an analog system increases, so does the price. Therefore, the ability to perform multiple tasks on a single chip saves the developer money. The other advantage to DSP is that it is by nature upgradeable. While analog devices must be completely redesigned to change their functionality, a DSP can simply be reprogrammed. This feature is very powerful, allowing developers to

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easily add features to a product or increase the quality of existing algorithms. In many cases manufacturers provide these upgrades directly to the customer either for free or at a small fee.

There were several products that were introduced this year that are similar in nature or function to our proposed design. There were several Digital Audio Workstations released by Akai, Roland and Fostex which where in the under $1000 price range. These all have digital controls and digital processing and at least 2 mic inputs. They all have some sort of digital recording built in and most have an option for a CD burner. However none of them are battery powered. There was a battery powered analog mixer that was introduced. This 6 channel mixer by Nady has a very affordable price tag of $200.

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2. Design Process 2.1 Experimental Work

We experimented making PCB’s with our current facilities, we went through several iterations of planning but developed a way of making double-sided PCBs using two transparencies separated by another PCB as a spacer. Along with experimentation with making double-sided PCB’s, we also had some traces that needed to be quite small (Ultiboard code 1). After attempting different boards with these traces we determined that we could not get consistent results with the processes available to us. Our final solution to our PCB fabrication difficulties was to send our designs to another company for fabrication, Alberta Printed Circuits (www.apcircuits.com), where all of the traces could be made reliably as well as the drilling and through holes. 2.2 Design Segments 2.2.1 Analog

The analog mixer circuits were first tested with the MultiSim software package. This testing, at the end of the design process, was used to double-check component values and confirm the frequency response of the equalization filters. Component values were adjusted, in this process, to move the center frequencies of the filters to those desired: 80 Hz, 2.5 kHz, and 12 kHz. Other components were adjusted to achieve a gain range of positive or negative fifteen decibels for each filter frequency.

The equalizer/filtering section of the mixer yields zero gain or attenuation of the frequency bands when the (linear taper) control potentiometers are centered. At full clockwise rotation, they each yield fifteen decibels of gain at their center frequency when used independently, but there is a small effect when they are used together. This effect slightly affects the gain range possible for each frequency band. At full counterclockwise rotation, each frequency band is attenuated by fifteen decibels—again, with a small effect when the different bands are used together. 2.2.2 Digital

The digital design of our project started our choice of a DSP. We initially chose the Motorola DSP56367. This processor has a multitude of I2S inputs which allow us to input serial digital audio data. We also liked the low power consumption of the DSP56367 compared with its siblings and believed that using a Motorola processor would be beneficial as we already had some experience working with Motorola assembly languages. As a result of this choice in processor, the rest of the chips that we would be using needed to have 3.3V inputs and outputs. This requirement led us to choose the TI TLC320AD77C as our ADC and DAC as it was the only readily available chip that we could find with the specifications we needed and 3.3V operation. Voltage requirement also

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led to our choice of the Analog Devices AD7829 as an ADC for the digital level controls.

As we entered the spring semester, we found ourselves unable to find any way to get a DSP56367 as Motorola does not give samples and the suppliers we contacted did not have it in stock and could not offer us a reasonable price because they would need to order a large quantity. Since the DSP56367 was unavailable, we chose to use the Motorola DSP56362 in its place. The DSP56362 has fewer inputs than the DSP56367 and a higher power draw but though it was not our first choice, it would be sufficient to our task. With our components settled on, we began the task of circuit layout. The first board that we layed out was for the TI ADC/DAC that we were using. After laying out the board we attempted to make it and discovered that because of the small size of the traces we needed to use, we could not make a PCB for this circuit or for the DSP board (which would need the same size traces) at our facilities. After some discussion and consultation with Steve Dove, we decided that the best option would be for us to send files with our design to Alberta Printed Circuits and have the PCBs fabricated by them.

Creation of the PCB for our DSP began with the attached connection diagram from Steve Dove. From this base, we created a circuit that was adapted to our purposes with ribbon connectors to connect the DSP to our other circuits. After sending multiple copies of our “final” layout to APC, we finally produced a layout that they would be able to produce and received our PCBs three days later. More details about the difficulty encountered in our layouts is covered in section 2.3.

When we were able to test our circuits, we were unable to produce any output from the ADC/DAC board. We were unable to determine the cause for this problem, but on looking back at our choice of chips, we realized that by making the change to the DSP56362, we could now use standard 5V chips for our ADC and DAC. With the voltage restriction removed, we turned to the Crystal CS5396 for our ADC and CS4390 for DAC. We had received samples of both of these chips in DIP form and were able to get them working together on a breadboard within a few hours. Because all of the chips needed were now in DIP form, we were also able to fabricate the PCB for these chips on our own as well.

The final portion of our digital design was the digital output. For this, we intended to use the Crystal CS8427 which would take the same I2S output from the DSP that the analog output used and format it to S/PDIF as we had specified. Unfortunately, when we connected the circuit we had created for this to the digital signal we had from the ADC, we were unable to get an output because the CS8427 was designed to be used in conjunction with a controlling processor. Because we had not intended to run this chip with DSP control, we had not provided access to the lines needed to control it. Again on advice from Steve Dove, we chose the Crystal CS8402A which is a discontinued product that Steve Dove provided us with samples of.

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2.2.3 Other Design parameters for the power supply initially had few specifics; it had to supply all the necessary voltages, provide enough power for all devices, and operate from a battery. Further inspection of power requirements listed for each component revealed that the supply needed to have two individual voltages for the digital devices, 3.3 and 5 volts, and two voltages for the analog devices, negative and positive 12 volts. For noise considerations in the analog devices and proper operation of the digital devices a ripple constraint of 2 µV. As with any battery-powered appliance, efficiency is a cardinal law and in any audio appliance, no noise is considered the Holy Grail for operation. The specifications for the power supply became the following: supply four voltages, 3.3, 5, -12, and +12, at 2µV ripple for at least two hours with a respectable efficiency. After a through review of current industry standard DC-DC chips and conversion methods the circuit simplicity and specifications of the DATEL BWL-12/830-D12A and BWL-5/6-3.3/7-D12 power blocks were chosen. Inspired by the wastefulness of ITS and the need for twisted pair to reduce noise in power distribution, CAT-5 ethernet jacks were used to create the quick connect power distribution network. (Please note all schematics and pictures in appendix.) A VU meters purpose is to give the operator a visual reference of the sound level for signal integrity and equipment safety. The specifications on this device were to display a stereo signal, have a realtime response, conform to the American standard of average signal metering, and circuit simplicity. The LM 3916 LED driver chip met all of these specifications. In single chip modes, the display would allow for an audio range of –20 to 3 dB in ten increments to be displayed in realtime using average signal metering with one chip and four common components, minus the LEDs. 2.3 Twists and Turns

Our biggest challenge was working with the small trace and component sizes our circuit boards needed. Given the small scale and number of components, fabricating many of the PCB’s with our current facilities was not an option. This led us to out source many of our boards that had these small components. This added expense allowed us limited design iterations. To allow the most flexibility in our designs, we fabricated our mixer from multiple PCBs with ribbon cable interconnects. This modular approach allowed for easy future upgrades and sectional fixes as well as simplifying individual board construction. In our initial design, our contingency plan was to cut out the digital mix stage and have an analog mix stage with a digital output. Needless to say, late in our construction stage, we experienced problems with our DSP. A word to the wise, pin 1 is really marked with a dot. We accidentally put our DSP

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in wrong, and there is a very good possibility that we cooked it. This is not necessarily the case, but at the point where we started having problems it was late enough in the construction phase that we decided to revert to our contingency plan.

The final construction came down to the wire. Even though we worked hard all year long, we needed one big final push at the end to get everything together. The last month before the presentation we each spent around 35 hours a week working on the project and the week of presentations, all other class work ground to a halt as we brought all our pieces together. All our individual designs were working and the final step was to put them together, John Meyer and various other people helped with the physical case construction for which we are extremely grateful. Since all our parts were working individually we didn’t foresee any major hang-ups. However, Murphy’s Law still holds true. When we finally got it electrically all together at 2 AM on the day of the presentation, nothing was working properly. The A/D wasn’t initializing properly and the analog signal chain was experiencing serious noise and distortion problems. It was only by grace at this point that we didn’t resort to violence or do something we would have regretted later. We spent several hours’ trouble shooting and around 5 AM got the digital section working again. It was at this point that we decided to take a nap and reconvene at 10 AM. After a few hours of sleep, we decided to pull pre amp functionality leaving us with a line mixer, this move worked, leaving us with a working mixer. By 2 PM we had a completed project in time for a shower and another rehearsal before presenting our project.

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3. Implementation 3.1 Construction 3.1.1 Analog

The first step in board construction is component and copper trace layout. This was a very time-consuming task, especially for the channel strip board (which includes the preamplifiers, equalizing filters, and peak indicators for all four input channels). Component and netlist files were imported into Ultiboard from Multisim, and then the components were moved, and traces drawn and redrawn, until the layout worked from an electrical and mechanical perspective. The layout was printed out and then transferred to a transparency for the making of the actual board.

The analog boards for the mixer were fabricated using photosensitive copper-clad boards. Using a transparency printout of the desired traces and pads to mask them, the boards were exposed to light, removing the photosensitive covering from areas from which we needed the copper removed. The boards were then etched with a solution that removed the copper that was not covered by the photosensitive coating. This left boards with the desired copper traces and pads, without the rest of the original copper clad. Once the boards were made, a hole had to be drilled in the center of each pad so that components could be mounted to the boards. When this was finished, the components had to be soldered onto the copper pads. As each component was soldered, it was very important to ensure that the connections were good (not cold soldered) and that the solder did not connect separate traces that were not supposed to be connected. External controls such as switches and potentiometers were an especial challenge, since the wires had to be measured individually for length and because the need for mounting wires meant that there were twice as many solder connections. A final step in construction of the analog boards was to drill mounting holes to allow the boards to be fastened into the finished mixer. 3.1.2 Digital

As mentioned in the design section of this report, one of the most difficult construction tasks was fabricating the PCBs. These challenges started with the circuit layout. Because we would not be fabricating the circuit boards ourselves, it would be much cheaper for all of the layouts to be in a single file. In order to do this we needed to import multiple groups of components and netlists into a single file. Unfortunately, Ultiboard does not add a second import to the first but rather replaces the first with the second. Later in the process, we discovered that Ultiboard does have group export and group import functions that would have allowed us to create the circuits in multiple files and imported all of them into a single file to be sent to APC.

The other challenging aspect of the digital construction was a design challenge as well. Because many of our chips were in surface mount packages,

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we were unable to do any sort of testing before creating the PCBs. The lack of testing before we made the final circuits meant that testing had to occur after the PCB was created. Unfortunately, we discovered that many of our initial designs were not perfect and the first and even second designs did not work so a large part of construction became testing and redesign. Unfortunately, there was one area where this was not possible, the board for the DSP would have to be perfectly designed and perfectly constructed. We will never know, however, if the DSP board was correct (though we believe it is) because on our first attempt at testing, we put the chip in its socket upside down and burned it out. 3.1.3 Other

Construction of the power supply was fairly straight forward. Care was taken not to short any of the ground to the power lines being that their circuit traces were so close together. Though the power blocks were equipped with a safety mechanism to prevent any damage to their circuitry, it would be difficult to diagnose any problems in the other devices if such a short occurred. After careful construction and placing the proper fuses onboard to satisfy the paranoid designer, the board was tested and it performed to desired specifications with a single 12-volt battery source. Construction of the VU meter was complicated only in attempting to create the simplest symmetric geometry as possible. After an acceptable design was completed, it was etched and built. The circuit operated as designed and exceeded the design requirements desired for it. (Contrary to purported theory, circuits can be designed and built without testing and work the first time around. George did it three times in a row. Be encouraged.☺) 3.1.4 Physical

For our mixer’s enclosure we chose to use a stock aluminum box and drill holes where necessary for all of the knobs, jacks and switches. The most complex part of the mixer would be the top which was made from a separate aluminum panel and would need approximately 75 holes drilled with a great deal of precision. In order to achieve the precision we needed, we created an Ultiboard file with pads representing each of the holes we would need and created a drill file for the CNC milling machine. From this John Meyer drilled our aluminum plate for us. 3.2 Operation The analog sections of the mixer worked well when tested individually. However, we had some difficulties with them when they were integrated in the finished mixer unit. Despite these difficulties, we were able to demonstrate the functionality of the overall mixer. The preamplifiers did not work in the prototype mixer. However, the equalizing filters worked beautifully. The peak indicators were disconnected in the final prototype because they were not

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working properly. We do not yet know why the peak indicators were not functional, because their design was not changed from a design that worked very well. The analog mixing board worked very well overall, though some of the panning controls gave some difficulty. The output drivers worked perfectly throughout the design and construction of the mixer. The problems encountered in the analog sections of the mixer are particularly interesting because they were not consistent from channel to channel—the first channel of the mixer worked beautifully throughout, while the others gave problems despite being identical in design to the perfectly functioning channel.

Many of the digital aspects of our mixer are not currently functional. Most notably, our final design has no DSP. Due to an unfortunate error, we inserted our DSP upside down and consequently burned out the processor. Though the DSP itself did not work, we believe that our circuit design is valid and would work with a new DSP. We also have code that has been debugged in a simulator and should work with few if any modifications should another group continue this project. The other digital circuit that does not work at this time is the controls board. Though we were unable to determine the reason for this with certainty, we believe that it is due to a faulty address decoder chip. Due to difficulties with Ultiboard (when you put a surface mount chip on the bottom of the board it fails to reverse the pins) we were forced to bend all of the pins on this chip the opposite way which we believe has led to its malfunctioning. Finally, the last non-functional digital circuit is the expansion bus circuit.

While there are digital circuits that do not currently work, the most important circuits have been completed. The ADC circuit is functional and has only one issue to be addressed. The Crystal CS5396 requires that a calibration signal be sent to the chip at some point after all of the voltages have settled which occurs about 12-15 seconds after the chip is powered on. We have not yet created any circuit to perform this function and have only wire leads that we manually touch together at the appropriate time. Also functional is the digital output driver though this circuit too requires a delayed calibration signal, it could be derived from the same source as the ADC. Finally, the DAC is functional.

In addition to functioning on their own, we were able to test these circuits’ ability to work together. We tested both the DAC and the digital output circuits by tying their inputs to the output of one of the ADCs. When we did this, we were able to hear the signal from the DAC as well as monitor it on the oscilloscope and observed some attenuation at about 10kHz and above. We were also able to make a digital recording using the digital output circuit. On comparing the recording we made to the original signal, we were quite pleased with the results. Though we were able to detect some tonal differences in the two signals, they were minor and could be the result of the previously mentioned attenuation.

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4. Schedule 4.1 Gantt Chart See Attached 4.2 Explanation

The primary delay in our schedule was the time involved in circuit board layout. Though we knew this would take a good amount of time, we spent a great deal of time adjusting our design so that it could be produced by APC. Our schedule was also affected by a late start in the second semester. Because of our demanding classes and the fact that we did not assign a group leader, we had a great deal of trouble getting together to organize our efforts on our return to school.

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5. Budget 5.1 Prototype vs. Projected Production costs

The sum total of our parts including samples came out to approximately $653.00. Since we essentially had two different designs there would be a disparity between the cost of the Mixer with a digital mix stage and an analog mix stage. Regardless of the production model, these costs would drop dramatically in quantity and some parts would become unnecessary. The actual final price of the unit would be some factor of the cost of the materials.

Prototype Gifts in Kind Digital Mix Stage Analog Mix StagePC board 150.00 1.00 1.00DSP 22.00 15.00 0.00DSP Socket 70.00 0.00 0.00Codec - 2 5.00 20.00 20.00S/P DIF driver 5.00 3.00 3.00Toslink 5.00 3.00 3.00Analog Electronics 50.00 30.00 35.00Potentiometers 41.00 20.00 20.00Jacks 15.00 7.00 7.00Case & Hardware 30.00 15.00 15.00Power Electronics 170.00 50.00 50.00Battery 20.00 20.00 20.00Misc Shipping 70.00 0.00 0.00Totals 568.00 85.00 184.00 174.00

The cost of labor and other non material costs.

Units Cost per Unit TotalLabor 1000 $40 $40,000Independent Consultant 1 $60,000 $60,000Example Code and Layout 4 $5,000 $20,000

The cost of finishing our senior project and graduating: Priceless.

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6. Conclusions 6.1 Objectives Met 4 microphone/line level inputs with a signal to noise ratio of 90 dB

Untested – preamps are not functional on the final board

A 3 band equalizer on each channel allowing ± 15 dB of gain at the chosen frequencies

Met

A digital output adhering to the S/PDIF specification of the main Left/Right output channels

Met

An expansion bus adhering to the I2S specification that allows the ability for additional devices to be attached directly to the DSP.

Not yet implemented

Battery operation for at least 2 hours Met 6.2 What We Learned

• Though surface mount chips are becoming more and more common, being able to breadboard the circuits you make is irreplaceable. If you have no choice but to use a surface mount chip, make an adapter board early so that you can work with the circuit before you do all the work involved in making an entire PCB.

• Before designing a custom circuit, check to see if any chips are available to do the job. You can save weeks or months of work if you can use a pre-designed chip in place of an untested idea—and quality may be higher, as well.

• Read the entire datasheet. We had one datasheet in particular where information important to the mode that we were using the chip in was only given in the section for the other mode.

• Pick a leader. A group without a leader will pay for that decision (or lack thereof) the entire year as there will be nobody to organize meetings, communicate with your advisor, keep track of the budget, etc.

• You can never plan enough time for circuit board layout. This is a skill that takes practice and while you may be able to do complex layouts in an hour by the time you’re done with the project, you can’t when you start it.

• Remember to plan extensive time for drilling and soldering in any electronic project. Complicated boards don’t take any longer to fabricate, but drilling hundreds of holes and soldering hundreds of leads can take a significant amount of time.

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• Test often the interaction between separate circuits in the design and prototyping process. Otherwise, when separate boards are integrated in the end, they may not work as well as they did individually.

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7. Recommendations for Future Work. Purchase DSP and debug code and Controls board. Create a circuit board and implement code for the expansion bus. Fewer circuit boards, this would increase signal integrity and decrease

production costs. More digital controls and data display would allow for added software

features such as compression, parametric eq and virtual channels. Digital inputs. Aesthetic considerations, an attractive box and well designed layout go a

long way to increase a products value and usefulness. Add Phantom Power. Implement devices on the bus: effects processor, additional channels.

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Appendixes See Accompanying Binder for Circuit Diagrams and Layouts Glossary A/D Converter (ADC)

An A/D (Analog to Digital) converter is an electronic device whose function is to convert analog voltages into a binary representation that can be stored and manipulated, and later retrieved or converted back to analog.

Balanced

Balanced refers to a differential type of AC electrical signal having two "legs" independent of ground. One is generally considered positive (+) and the other negative (-) in voltage and current flow with respect to ground. Unlike unbalanced audio lines, there is no "signal" carried in the shield or ground connection unless there is a fault. The main benefit is that any noise that gets induced into the line will be common to both the positive and negative sides and thus canceled when it arrives at its destination, assuming the destination is balanced. This phenomenon is called "Common Mode Rejection" meaning that any signals common to both the positive and negative legs of balanced lines will be removed. This happens because when the receiving device looks at the signal the common noise actually shows up as out of phase with itself, and gets cancelled. Think of it as if the negative (-) signal gets inverted to positive (+) before use, which puts the desired audio signal in phase with the already positive other leg and at the same time causes the undesired common noise to become out of phase with itself.

D/A Converter (DAC)

(Short for Digital to Analog Converter) Also often abbreviated DAC, which is often pronounced "dack." A DAC is the opposite of an A/D converter. It reassembles the digital ones and zeros back into an analog waveform. One of the key ingredients in this process is filtering, which is what makes the final analog result a smooth (as opposed to digitally stair stepped) waveform. DACs come in a variety of configurations and price ranges.

dB

(Abbreviation for Decibel) It literally means one tenth of a bel. The bel is a unit of measurement named after Alexander Graham Bell. The bel had its origin in the Bell Telephone Labs, where they needed a convenient way to express power losses in telephone lines as power ratios. Because the bel is

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a power ratio of 10, and this is a rather large ratio, it is convenient to divide it into tenths of bels, or decibels.

dBu

Abbreviation for decibels as referenced to voltage. In this case the zero reference (0 dBu) is .775 volts, but does not assume any load impedance. A rating of +4 dBu in equipment specifications implies a reference or nominal voltage of 1.23 volts. The formula for calculating dBu is 20log (V1/0.775) where V1 is the voltage being measured. With this you can see that 20log (1.23/0.775) = +4 dBu.

DSP Abbreviation for Digital Signal Processing. Without getting into a lot of

detail it refers to a specific type of digital processing that is optimized for dealing with signals. In our case these are audio signals, but they don't always have to be. DSP chips are designed and optimized to be able to do various (mathematical) calculations for processing audio or image data.

EQ (Equalizer)

Based on the root word, equal, an equalizer is an audio device whose function is to equal out the tonal characteristics of a sound. First conceived as a tool used to get flat response in telephone lines and to make up for the deficiencies in audio equipment and acoustic spaces, they are more often used creatively to alter the relative balance of frequencies to produce desired tonal characteristics in sounds. An equalizer has the ability to boost and/or cut the energy (amplitude) in specified frequency ranges by employing one or more filter circuits.

Resolution

Resolution is a measurement of the fineness of detail captured in a representation of something. In digital audio resolution is affected by the sampling rate and the bit depth of the recording: 24-bit audio has a higher resolution than 16-bit audio, and a 48 kHz sample rate has more resolution than a 44.1 kHz sample rate.

S/PDIF

A format for interfacing digital audio equipment together, S/PDIF (Sony/Philips Digital Interface Format) is considered a consumer format, and is largely based on the AES/EBU standard. S/PDIF typically uses either unbalanced, high impedance coaxial cables or fiber optic cables for

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transmission and allows for digital signals to be passed from one device to another.

Sample Rate

In a digital recorder or sampler, the sample rate is how many times per second the source material is being "sampled" or recorded. Sample rate affects the frequency response of the final recording or sample; the highest frequency that can accurately be sampled is 1/2 the sample rate. In general, the higher the sample rate, the better the sound quality. CDs use a 44.1 kHz sample rate, while DAT recorders often default to 48 kHz. Multimedia applications may use rates of 22.05 kHz or even 11.025 kHz for maximum efficiency.

THD (Total Harmonic Distortion) THD is the ratio of the power of the fundamental frequency at the output of a device versus the total power of all the harmonics in the frequency band at the output of the device. Basically, all electronic audio devices introduce some distortion to audio passed through them. The simplest form of this distortion is the addition of harmonics to the outputted signal. THD represents the sum of all the harmonics added by a device as a percentage of the level of the signal being measured. The closer THD is to zero, the more "transparent" a device should.

TRS

(Abbreviation for Tip Ring Sleeve) This is the descriptively accurate term used to describe 1/4" (or 1/8") balanced connectors. A TRS plug can be found at the end of most headphone. They look like a standard 1/4" plug with three sections in them. The three sections of the shaft are called the Tip, Ring, and Sleeve. TRS connectors are used wherever it is desired to have two conductors plus a ground in one plug. Common uses are as a way to connect balanced equipment (where the TRS plug has a positive, negative, and ground connection), or unbalanced stereo equipment (left and right are on the Tip and Ring, with a common ground) like headphones, or as an insert for your mixer or other processor (Tip or Ring is the send with the other being used as the return and again ground is common).

XLR Trademarked name for circular 3-pin connectors developed by Cannon (now owned by ITT). "XLR" was originally nothing more than Cannon's part designation for the connector. In fact, you'll also sometimes see these connectors referred to as "Cannon" connectors. XLR has since evolved into a generic term. In audio work, XLR connectors are normally used for

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transmitting balanced microphone- and line-level signals. Pin 1 of an XLR connector is always ground/shield. The connectors are designed so that pin 1 makes its connection first when inserted in a jack; this ensures that the ground connection is made first, helping prevent pops and thumps in the audio chain.

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