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Don't trust everything you read on this forum

Posted by Christine Tham (R) on July 13, 2007 at 20:46:52

In Reply to: Correct on all points, Christine, but compression has been a fact posted by sgb on July 13, 2007 at 06:01:23:

The statement "dynamic compression is as old as stereo sound" is not true, at least not the sort of dynamic compression that is now applied on recordings.

And your extrapolation that "compression has been a fact of life long before the advent of stereo" is even further from the truth.

Techniques such as peak limiting has been used since the early days of recording (since tubes and magnetic tape have peak limiting characteristics when stressed) but this is a different kettle of fish from the techniques applied today, because digital recording have hard limits for maximum amplitude, unlike analog.

*** how important it is to have at least 18 dB headroom ***

it's actually not headroom, just simple maths. Uncompressed music has dynamic range that can exceed 130dB, a good recording probably captures about 120dB. By contrast 99% of systems would struggle to reproduce more than 110dB of usable dynamic range.

So there's typically at least 10dB (or about an order of magnitude difference in recording levels, since the dB scale is logarithmic) between what can be captured and what's realistically playable.

If you don't believe me, send a low frequency signal to your speakers, and measure the voltage using a voltmeter. It will peak and start clipping long before you get anywhere near 110dB. For example, on a amp rated at 100w with reasonable headroom, don't be surprised if it starts clipping as low as 40w. This is because the power rating of amps can be a misleading measurement (the "power" of an amp is measured at 1% THD when the amp is well and truly overdriven and exhibiting gross clipping - typical THD is more like 0.01%). In fact to get an amp that truly delivers 100w without clipping you may need something with a rating of 300-400w.

Someone else mentioned tubes clip differently than solid state. This is true, and the reason why many prefer tubes. The behaviour of a 20w tube amp amplifying a loud signal can result in a far cleaner signal (more "rounded" waveform) than a 100w solid state. But it's still clipping - in fact more so - it's just that the results are far less harsher to the human ear (due to the peak limiting characteristics of tubes when overdriven - something that early recordings often took advantage of - hence the "warm" sound).

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