A Clean Audio Installation Guide™

8.0 PROPER OPERATION
Many audio operators find themselves fighting a perplexing and confusing battle with audio amplitudes. This often is because of the inability of the VU meter to show peaks of program material, which are the source of over modulation, tape saturation, amplifier and converter clipping. The VU meter, while being a universal measurement device in the Americas, Japan, France, and Australia,⁽¹⁰⁾ finds less application in the rest of the world because of this problem. The problems occur when adequate headroom has not been designed into the system. The late Hans Schmid of ABC in New York has shown⁽²⁾ that use of the peak program meter will give clearly superior indication of program material without lowering average amplitudes, but with a new control over excessive program peaks. In order to observe peak program amplitudes, it is desirable to convert existing VU meters to operate as a PPM as well as VU by the use of a conversion card, or add parallel PPMs along side the VU meters. As a bare minimum, at least add peak overload indicators of some form to any equipment without that capability.

9.0 TESTING
Overall noise and frequency response measurements should be taken. It is our firm conviction that if the above procedures are followed, wide band (f₃ =100 kHz) audio systems (depending upon the number of pieces of equipment) right up to the band limiting device, such as an A-to-D converter, stereo generator, or pre-emphasis network, are feasible. This allows the band limited device to set the overall system bandwidth. This, as we have seen, allows for a flatter frequency response (not counting intentional response shaping) rather than allowing an accumulation of 3 dB roll-off points to upset the high frequency response and a proof-of-performance test in broadcast systems, for instance. It allows low phase shift out to 20 kHz and good transient response, provided the various pieces of equipment have proper feedback compensation.

9.1 Square Wave Testing
We recommend that you do extensive 10 kHz square wave testing of the system by placing a function generator at the start of the audio chain and then look at every output with your scope. We recommend that all response type measurements be made at -12 dBu. This conforms to the new CCIR recommendations.⁽¹⁰⁾ For square wave testing, it ensures that no slew rate limitations will confuse the picture of system bandwidth. The -12 dBu level makes sure the amplifiers under test do not go into saturation during this test; otherwise, any accuracy in the conclusions is null and void. Check for overshoot or ringing, which would indicate under-damping of amplifier circuits, and thus potential instability and RF susceptibility. Do this test with each piece of equipment separately and then as a system. A small amount of high frequency related overshoot from an interconnect may be seen due to 60 ohm drive impedance. As discussed above, the 60 ohm figure is actually a compromise in favor of bandwidth and is slightly lower than necessary for a "maximally flat" response. There will be some HF peaking. Approximately 85 ohms would give the maximally flat response, but is restrictive of bandwidth. When you reach a stage that has significant bandwidth limitation, noted by the strong rounding of the corners of the square wave, it is time to move the generator up past this point in order to get a true indication of what is happening in the following stages. (See Ref. 7)

9.2 Distortion Testing
Once you are satisfied with the frequency response, noise floor, and transient response of your system, it is well to do some overall distortion measurements. While low frequency total harmonic distortion measurements have merit, in bandwidth limited systems, high frequency THD measurements are almost meaningless. The filters in an A-to-D converter or stereo generator do a nice job in removing the distortion products of anything above 10 kHz and destroy the accuracy of measurements much above 2 kHz; hence everything may measure better than it actually is. If slewing induced intermodulation and/or transient intermodulation distortion exist within the system, it will only occur at the higher frequencies. The best way in a bandwidth limited system to detect their presence is with the CCIF twin tone IM distortion measurement. By using 14 and 15 kHz tones mixed 1:1, a 1 kHz IM product is easily detected if SID/TIM exists in an FM broadcast system. A 19 and 20 kHz pair may be used to evaluate A-to-D converters or other equipment in the audio chain. We believe that every broadcast facility should perform twin tone IM distortion measurements to see the truth about their high frequency audio performance.

9.3 Useful Tools
Several tools useful in testing your system apart from your ears:

The telephone pickup coil, Radio Shack #44-533: with a pair of headphones and a microphone preamplifier, it is possible to do a thorough electro-magnetic survey of the space in which you intend to place your equipment. See the Benchmark Media Systems, " The Audio Microscope™" application note for an amplifier setup that will do the job.
A battery operated, wide-band, high input impedance AC voltmeter, with which you can measure the voltage differences between various pieces of equipment.

The ultimate in test systems, from our viewpoint, is the Audio Precision, System Two, Audio Precision, PO. Box 2209, Beaverton, Oregon 97075, 1-800-231-7350. This device's abilities would take pages to describe. See a review of the older System One by the late Hugh Ford in "Studio Sound", June, 1986. In addition to all of its distortion measurement capability, it has the ability to do an FFT analysis and thus give amplitude measurements of all the power line harmonics that may be present in your audio. This will allow you to discern between hum that is primarily from internal power supplies (second harmonic), and that which may be induced from the power mains (fundamental and third harmonic).

10.0 CONCLUSION
If you perform these steps with care, you will have a system that is capable of outstanding performance. It should have a residual noise floor not too far from that of the equipment with the highest output noise. In a broadcast facility, the satisfying of these criteria and its proof by the above measurements will allow you to complete a more meaningful set-up of your compressor-limiter, after which additional experimentation such as L-R processing can be performed. Hopefully, at this point, the engineering department and the production people will find harmony in their Clean Audio.

Go to: Section 11.0