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I feel that understanding the historical context surrounding the establishment of certain practices...
Now throw in that actually it was common to use +8 as referenced to 0db, rather than +4, and your starting to see why some equipment was built to pass +27 before clipping. Not sure why I remember seeing it as +27 and not 28?A link that I ran across a couple days ago, while written a long time ago for a different purpose, but talks to mic pre output levels might be of interest to you if you haven't seen it....http://www.dwfearn.com/tubes_vs_transistors.htmlEV3
This is all way over my head, but I thought I would ask Jim Anderson, whom I regard as one of the greatest recording engineers of recent times. (He did the best of Patricia Barber's recordings.)He didn't answer directly, but passed the question to a friend, who answered as follows, in favor of the +24 guidelines (I think!) . I post with their permission.Jim;Regarding the concerns of Arthur Ogus about Mr. Roger Modjeski's question that was posted on AudioCircle (http://www.audiocircle.com/index.php?PHPSESSID=g3cdpn6edidnuker3m7590gea0&topic=111645.0), here are my thoughts. You may forward these comments to anyone. I'm going into greater detail than you need, but someone else may need it or appreciate it (or disagree with it):From Jim Anderson:I always thought the +24 was about headroom and was very necessary and frankly thought +32 was preferred.As Mr. Modjeski explains, there are specific reference levels such as "0 dBu" (0.775 volts). "Headroom" must be expressed in relationship to a specific reference level, otherwise it is meaningless (Less McCann: "Tryin' to make it real — compared to what?").What is headroom? One way to describe it is to specify a signal level which becomes the reference level, then determine how much higher the signal level can get before the signal becomes unacceptably distorted. The difference between the two levels is the headroom.To put "headroom" into some context: For many decades there has been a traditional "operating level" of "+4 dBu" or "+4 dBm" (same voltage level but different loads being specified). This is 4 dB above the "0 dBu" or "0 dBm" reference level (0.775 volts). I am not a historian, and cannot speak for the folks at Bell Labs or anywhere else. But they likely chose a standard operating level that was high enough to be well above the residual broadband noise of the typical circuitry of the day, yet low enough that there was sufficient room above that level to handle higher signal levels caused by the dynamics of program material and unexpected peaks without excessive distortion. Enter the engineer, with the knowledge and experience to adjust levels during a performance to keep them in the sweet spot. The engineer's job was somewhat challenging because the mechanical VU meters of the day were "averaging" devices, incapable of responding quickly enough to indicate the true levels of transients.I have always calibrated the meters on my M-1 mic preamp so that the "0VU" point on the LED meter scale (the first 15 LEDs illuminated) represents an output level of +4 dBu in either the PEAK mode or the VU mode. The meter is indicating the output level of the preamp. This is how the old-fashioned mechanical VU meters were typically calibrated (0VU = +4 dBm). I specify the maximum output level of the M-1 as +24 dBu (it is actually capable of a maximum output level of +25.8 dBu under normal circumstances, but I give it a conservative rating of +24 dBu), so there is 20 dB of "headroom" beyond the standard operating level of +4 dBu. There would also be 24 dB of headroom beyond the reference level of 0 dBu. The "PK" LED is normally calibrated to illuminate when the output level reaches +22 dBu, giving you 2 dB of warning before you reach the maximum output level of the M-1 (+24 dBu). Therefore there is 2 dB of headroom between the point at which the "PK" LED illuminates (+22 dBu) and the maximum output level (+24 dBu). If you use the maximum output level specification of +25.8 dBu, you have 3.8 dB of headroom beyond the point at which the "PK" LED illuminates (+22 dBu) and the actual clipping point of the signal (+25.8 dBu). Similarly, you have 25.8 dB of headroom beyond the 0 dBu reference level, and 21.8 dB of headroom beyond the +4 dBu operating level. So there are several definitions of "headroom" given here, each one properly defined by specifying a reference level to which the higher level is compared.Quoting Mr. Modjeski:My current research leads me to believe that recording engineers still want +24 dBu levels which were desired in the days of analog tape but which make no sense in the current studio where the tape recorder has been replaced by digital recording via a A/D converter and computer. It appears that commercial A/D converters achieve full scale at +4 dBu or -10 dBu depending on settings. Given this there is no need to build to +24 dBu, hense the title of this post and an article I plan to write when I gather all the facts.I don't make A/D converters, so I can't comment with great experience regarding their optimum operating levels. I'm sure that there are many different topologies and approaches. Some A/D converters can accept signal levels as high as +24 dBu at their inputs without causing distortion during the A/D process. The A/D converter may even provide a higher S/N ratio when receiving a higher input level than a lower input level. A well-designed A/D converter should be able to deal with a +24 dBu input level without distorting, though it may require the ENGINEER to make the appropriate adjustment to the internal gain of the A/D analog circuitry. Each situation will be different, and each engineer or studio will have to make their individual choices on optimum operating levels. I would suspect that an A/D converter that reaches full-scale with a -10 dBu (-10 dBv?) input level would have a less-than-optimum S/N ratio, simply because a signal level of -10 dBu is relatively low, and the noise floor of the analog circuitry cannot magically be lowered to compensate.There will be situations where a particular combination of performer, microphone and mic preamp will result in output levels from the preamp that are well above +4 dBu, perhaps approaching +24 dBu, even when the gain controls of the preamp are set to the minimum gain setting. Yet the output level will not exceed the traditional "maximum" level of +24 dBu. You can't tell the artist to sing or play more softly or to back away from the microphone because you can't exceed the maximum input level of +4 dBu for a particular A/D converter. Your A/D converter must be able to deal with these real-world situations of high input levels.Many years ago I had a customer that had an A/D converter (that shall remain nameless) that had power supplies of +/-5VDC for the analog input stage. Input levels beyond about +8 dBu would cause clipping. Even worse, there was a volume control AFTER the analog input circuitry, and some type of meter AFTER the volume control. When the output of the M-1 exceeded +8 dBu, the input stage of the A/D converter started clipping. But the volume control of the A/D was turned down so that the meter of the A/D said that the level was OK. Incredibly dumb design. Perhaps the A/D was designed for people to convert their vinyl record collection to digital. I don't know.This particular A/D converter was capable of handling input levels at least a few dB greater than the +4 dBu level referred to by Mr. Modjeski. But it was woefully inadequate in the real world. Each engineer will have to make his or her own decision on how much headroom is enough for their situation. But, all else being equal, more headroom is better. Sooner or later, it will save the day. It costs more to provide more headroom. Power supplies must provide higher voltages. Transistors must be rated for higher voltages. Capacitors must have higher voltage ratings and they will be physically larger as a result. Power transformers will have to be larger, etc. The "-10" reference level was likely the result of attempts to lower the costs of audio equipment while still providing performance that was "good enough" for the typical homeowner or beginner or whatever. But when you make your living making recordings where reliability is essential, and you need to do everything that you can to avoid clipping, the additional cost of headroom is worth it. The tools that we build should make the engineer's job easier, not harder. Signal levels that are far above +4 dBu, even approaching +24dBu, are real, common, and they are not going away.Thank you.John HardyThe John Hardy Companywww.johnhardyco.com