Hear Hear! ;D
David, Frank,
Amen, Amen, Amen.
There is a LOT to be said for what you describe, though I have never found a paper to refer users to on this subject. Years ago I read an article about this in Broadcast Engineering.
Readers - here's an over view. Every piece of analog gear has an output "clip point" and an input "overload point" (maximum input level). They may or may not be the same!
Sometimes this information is available from the manufacturers spec sheet, but not always. Since the most common spec given is "nominal level" the issue of headroom comes into play.
The nominal output level of a device + headroom = overload point.
The nominal input level of a device + headroom = clip point
If the clip point of an output is matched to the overload overload point of the following input, maximum usable dynamic range is now possible. Any other scenario is non-optimal in terms of noise and/or distortion.
Complicating this scenario is that it became popular among board manufacturers to play specmanship with output headroom specs. Ostensibly this is/was due to the problem of sloppy board ops "clipping the board's outputs". However this "solution" to sloppy board ops was no solution at all, since the input to the next device would become over loaded (when board ops were sloppy) instead of the board clipping. Same result, but allows the board manufacturer to "point the finger elsewhere" as to the cause of the distortion.
Here is an example of the problem, and the solution:
Board has a nominal output of "+4 dBu" with a 21 dB headroom (e.g. clip point of +25 dBu).
Processor has a nominal input level of "+4 dBu" with 18 dB headroom (e.g. max input level of +22 dBu).
Problem - "Sloppy Jocks" can cause 3 dB of clipping in your air chain prior to the audio entering your processor. No AGC in the processor can fix this problem, because the audio is clipped at the INPUT stage to the processor.
Solution = A 3 dB pad between the board and the processor. The board ops may still be able to cause clipping, but the ONLY solution at that point is to teach them not to do it.
The reverse case also occurs, and means a few dB extra noise, but this case is not as easily remedied.
This stuff can really make a difference, particularly if the "operators" don't know how to operate the equipment. Sadly, as fundamental as this is, and as important as it is, it has become "lost knowledge" which is why I wanted to elaborate on David and Frank's comments.
And yes, engineers of lore used to painstakingly go through the entire airchain to eliminate these sources of lost headroom to PREVENT the GIGO effect going into the processing.
Final note: These concepts come into play with regards to "level discrepancies" when interoperability of digital equipment is desired.
For example, lets take a "budget codec" with inputs and outputs that have nominal +4 I/O with 15 dB of headroom (e.g. clip point and max input of +19), talking to a "hi-fi" codec with inputs and outputs with nominal +4 I/O and 18 dB of headroom (e.g. clip point and max input of +22 dBu )
For our analysis we will assume that the manufacturers of each codec have made the input overload point = to 0 dBfs.
We feed a tone of +15 dBu into both codecs.
In the budget to hi-fi direction here is the analysis: at the budget codec's input, +15 dBu = -4 dBfs. At the Hi-fi codec this digital information is received and it indicates a signal of -4 dBfs. The analog output of the hi-fi codec thus is at +18 dBu (4 dB below its max output level).
3 dB of "Gain" has occurred. ???
In the hi-fi to budget direction here is the analysis: at the hi-fi codec's input, +15 = -7 dBfs. At the budget end this is received and indicated as -7 dBfs. At the analog outputs of the budget codec the signal is output as +12 dBu (7 dB below its max output level)
3 dB of "loss" has occurred. ???
The solution is to make sure each codec properly interfaced at its *own* end.
So the hi-fi codec output must be padded (if necessary) to NEVER overload whatever it feeds.
And the budget codec may need a pad so that whatever is feeding it will never cause its inputs to be over loaded.
IMPORTANT stuff folks. Read and understand these concepts! 8)
