dbx Type IV
TM
White Paper
504X
504X Digital Output Card
®
14
of analog recording which those who abandoned analog and jumped on
the digital bandwagon were either never aware of or simply took for grant-
ed.
Anyone who has ever used analog tape knows that you can “hit it hard”
without destroying the recording. The printed specifications of analog tape
don’t take into account the practical headroom available. The max signal-
to-noise specification of analog tape is measured by defining the “max” sig-
nal as the point where a given signal level and frequency produces a given
percent Total Harmonic Distortion (THD)—typically the level at which a 1
kHz signal produces 3% THD. In actual use, the signal can easily exceed
this “max” signal level by 5, 10, or even 15 dB on peaks, depending on the
type of signal being recorded, without unacceptable artifacts. High signal
levels can be tolerated (i.e. more headroom) at the expense of increased
THD which, incidentally, is often desirable as an effect, evidenced by the
renewed popularity of tube equipment.
The obvious conclusion is that analog recording actually has more use-
able dynamic range than the specifications seem to indicate. For example,
let’s say we’re recording a kick drum. If analog tape measures 55 dB from
the 3% THD point down to the RMS noise floor and the peaks of the kick
drum exceed the 3% THD level by, say, 15 dB and it still sounds good,
then we have 15 dB of extra useable headroom. Therefore, we end up with
70 dB of useable dynamic range. Throw in noise reduction and we push
into the 90-something dB dynamic range territory of 16-bit digital. This
explains why well-recorded analog master tapes make good-sounding
CD’s with no objectionable noise.
One main drawback of digital is that it inherently lacks this forgiving
and beneficial characteristic of analog recording. Although digital conver-
sion exhibits wide linear dynamic range, when you run out of headroom
for high-level signals, hard clipping or even ugly signal wrap-around
occurs, not to mention that A/D converters have their own nasty side
effects such as going unstable when their modulator is overdriven with
high-level signals.
This shortcoming of digital conversion has drastically affected the way
users operate their equipment. Users are paranoid of overdriving the con-
verter input and end up recording at lower levels to ensure that there is
ample headroom to allow for the large peaks that would ruin an otherwise
perfect recording. This, of course, compromises signal-to-noise perform-
ance since the signal is now closer to the noise floor. Because users of dig-
ital equipment have to be extremely careful not to exceed 0 dB FS (full-
scale), they must use peak-reading headroom meters. On the other hand,
Summary of Contents for dbx 504X
Page 1: ......
