Audio compression - The MPEG - page 105

of the second stage in dB and so on. The final result is obtained by adding together all the insertion losses in dB
and adding them to the input level in dB(u) to give the output level in dB(u). As the dB is a pure ratio it can multiply
anything (by addition of logs) without changing the units. Thus dB(u) of level added to dB of gain are still dB(u).
In acoustic measurements, the sound pressure level (SPL) is measured in deciBels relative to a reference pressure
of 2 x 10 -5 Pascals (Pa) rms. In order to make the reference clear the units are dB(SPL). In measurements which
are intended to convey an impression of subjective loudness, a weighting filter is used prior to the level
measurement which reproduces the frequency response of human hearing which is most sensitive in the midrange.
The most common standard frequency response is the socalled A-weighting filter, hence the term dB(A) used when
a weighted level is being measured. At high or low frequencies, a lower reading will be obtained in dB(A) than in
dB(SPL).
[ 2 ] Martin, W.H., Decibel - the new name for the transmission unit. Bell System Tech. J. (Jan.1929)
4.3 Audio level metering
There are two main reasons for having level meters in audio equipment: to line up or adjust the gain of equipment,
and to assess the amplitude of the program material. Gain line-up is especially important in digital systems where
an incorrect analog level can result in ADC clipping.
Line-up is often done using a 1 kHz sine wave generated at an agreed level such as 0 dB(u). If a receiving device
does not display the same level, then its input sensitivity must be adjusted. Tape recorders and other devices
which pass signals through are usually lined up so that their input and output levels are identical, i.e. their insertion
loss is 0 dB. Lineup is important in large systems because it ensures that inadvertent level changes do not occur.
In measuring the level of a sine wave for the purposes of line-up, the dynamics of the meter are of no
consequence, whereas on program material the dynamics matter a great deal. The simplest (and cheapest) level
meter is essentially an AC voltmeter with a logarithmic response. As the ear is logarithmic, the deflection of the
meter is roughly proportional to the perceived volume, hence the term Volume Unit (VU) meter.
In audio recording and broadcasting, the worst sin is to overmodulate the tape, the ADC or the transmitter by
allowing a signal of excessive amplitude to pass. Real audio signals are rich in short transients which pass before
the sluggish VU meter responds. Consequently the VU meter is also called the virtually useless meter in
professional circles.
Broadcasters developed the Peak Program Meter (PPM) which is also logarithmic, but which is designed to
respond to peaks as quickly as the ear responds to distortion. Consequently the attack time of the PPM is carefully
specified. If a peak is so short that the PPM fails to indicate its true level, the resulting overload will also be so brief
that the HAS will not hear it. A further feature of the PPM is that the decay time of the meter is very slow, so that
any peaks are visible for much longer and the meter is easier to read because the meter movement is less violent.
The original PPM as developed by the BBC was sparsely calibrated, but other users have adopted the same
dynamics and added dB scales, Figure 4.7 shows some of the scales in use.
In broadcasting, the use of level metering and line-up procedures ensures that the level experienced by the
viewer/listener does not change significantly from program to program. Consequently in a transmission suite, the
goal would be to broadcast tapes at a level identical to that which was obtained during production. However, when
making a recording prior to any production process, the goal would be to modulate the tape as fully as possible
without clipping as this would then give the best signal-to-noise ratio. The level would then be reduced if necessary
in the production process.
Unlike analog recorders, digital systems do not have headroom, as there is no progressive onset of distortion until
convertor clipping, the equivalent of saturation, occurs at 0 dBFs. Accordingly many digital recorders have level
meters which read in dBFs. The scales are marked with 0 at the clipping level and all operating levels are below
that. This causes no dificulty provided the user is aware of the consequences.