Using Lossy Data Reduction In The Studio; About Transmission Levels And Metering - Orban OPTIMOD 5750 Operating Manual

Fm/hd/dab+ digital audio processor

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Orban 5750 Technical Manual

Technical Data

Using Lossy Data Reduction in the Studio

Many stations are now using lossy data reduction algorithms like MPEG-1 Layer 2 or Dolby AC2 to increase the

storage time of digital playback media. In addition, source material is often supplied through a lossy data reduction

algorithm, whether from satellite or over landlines. Sometimes, several encode / decode cycles will be cascaded

before the material is finally presented to THE OPTIMOD-5750's input.

All such algorithms operate by increasing the quantization noise in discrete frequency bands. If not

psychoacoustically mask`ed by the program material, this noise may be perceived as distortion, an "underwater

sound," or other perceptual degradation. Psychoacoustic calculations are used to ensure that the added noise is

masked by the desired program material and not heard. Cascading several stages of such processing can raise the

added quantization noise above the threshold of masking, such that it is heard.

At least one other mechanism can cause the noise to become audible at the radio. THE OPTIMOD-5750's multiband

limiter performs an "automatic equalization" function that can radically change the frequency balance of the

program. This can cause noise that would otherwise have been masked to become unmasked because the

psychoacoustic masking conditions under which the masking thresholds were originally computed have changed.

Accordingly, if you use lossy data reduction in the studio, you should use the highest data rate possible. This

maximizes the headroom between the added noise and the threshold where it will be heard. Also, you should

minimize the number of encode and decode cycles, because each cycle moves the added noise closer to the

threshold where the added noise is heard.

Metering

Studio engineers and transmission engineers consider audio levels and their measurements differently, so they

typically use different methods of metering to monitor these levels. The VU meter is an average-responding meter

(measuring the approximate RMS level) with a 300ms rise time and decay time; the VU indication usually under-

indicates the true peak level by 8 to 14dB. The Peak Program Meter (PPM) indicates a level between RMS and the

actual peak. The PPM has an attack time of 10ms, slow enough to cause the meter to ignore narrow peaks and

under-indicate the true peak level by 5 dB or more. The absolute peak-sensing meter or LED indicator shows the

true peak level. It has an instantaneous attack time and a release time slow enough to allow the engineer to read

the peak level easily. Figure 1-2: Absolute Peak Level, VU and PPM Indications shows the relative difference between

the absolute peak level and the indications of a VU meter and a PPM for a few seconds of music program.

Studio Line-up Levels and Headroom

The studio engineer is primarily concerned with calibrating the equipment to provide the required input level for

proper operation of each device so that all devices operate with the same input and output levels. This facilitates

patching devices in and out without recalibration.

For line-up, the studio engineer uses a calibration tone at a studio standard level, commonly called line-up level,

reference level, or operating level. Metering at the studio is by a VU meter or PPM (Peak Program Meter). As

discussed above, the VU or PPM indication under-indicates the true peak level.

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