Current PDF Edition - Broadcast Dialogue

Worms from the can:
Audio pre-emphasis run amok!
T
o our list of things that seemed like a good idea at the time, but
which we’d really like to get rid of today, let us add FM preemphasis.
Designers of our FM broadcast transmission system were faced with a
problem; the noise level of audio received by FM increases dramatically
as frequency is increased. Actually, if left to its own devices, it has the
same spectral shape as white noise—double the frequency, double the
noise!
This was an early obstacle to FM as a high-fidelity medium.
Their solution was a reflection of the times. Audio from “natural
sources” tends to have much less audio in the higher octaves. And as
discussed here previously, in the analogue world any equipment in suboptimal
condition (whether it is an older microphone, a misaligned or
worn tape head or an old turntable cartridge and stylus) tended to result
in high-frequency roll off, further depressing the content “up high.”
The solution was to introduce pre-emphasis at the transmitter and
build in matching de-emphasis at the receiver. Boost the transmission of
the treble frequencies up out of the hiss, then roll off the highs a matching
amount during reception to bury that noise. This technique was easy
to apply and found its way into records and tape recordings, and even
early CD recordings as well. But its legacy has been a couple of recurring
problems; some easy-to-solve ones resulting from carelessness and at
least one more that is more subtle and hard to get rid of.
The simplest form of pre-emphasis involves a “hinge-point” followed
by treble boost of 6 dB/octave. The hinge-point is usually created by a
circuit with an R and a C, and when you multiply ohms and microfarads
you end up with a product in microseconds. And that’s why FM preemphasis
is referred to as 75 uS.
At 75 uS, audio at 10 kHz is boosted by almost 14 dB and, by the time
we’ve reached FM’s upper limit at 15 kHz, the boost is 17 dB—a lot of
boost!
European FM stations use 50 uS, which is much more moderate. Early
cassette tapes were 120 uS which is extreme but then so were the hiss
problems with standard cassette tape.
As station technicians, it is quite important to know where in the
program chain the audio is pre-emphasized and where it is not, and the
equipment makers have made it alarmingly easy to apply pre-emphasis
twice, which is never a good idea.
ENG
INE
ERI
NG
by Dan Roach
BROADCAST DIALOGUE—The Voice of Broadcasting in Canada • April 2012 36