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2-4
Installation
Studio-Transmitter Link
Transmission from Studio to Transmitter: There are five types of studio-transmitter links (STLs) in common use in
broadcast service: uncompressed digital, digital with lossy compression (like MPEG, Dolby®, or APT-x®), microwave,
analog landline (telephone / post line), and audio subcarrier on a video microwave STL.
STLs are used in three fundamentally different ways. They can either:
1)
Pass unprocessed audio for application to the 5750's input.
2)
Pass the 5750's peak controlled analog or digital left and right audio outputs.
3)
Pass the 5750's peak-controlled composite stereo baseband output.
The three applications have different performance requirements. In general, a link that passes unprocessed audio
should have very low noise and low nonlinear distortion, but its transient response is not important. A link that
passes processed audio doesn't need as low a noise floor as a link passing unprocessed audio. However, its transient
response is critical. At the current state of the art, an uncompressed digital link using digital inputs and outputs to
pass audio in left/right format achieves best results. We will elaborate below.
Digital Links: Digital links may pass audio as straightforward PCM encoding or they may apply lossy data reduction
processing to the signal to reduce the number of bits per second required for transmission through the digital link.
Lossy data rate reduction will almost invariably distort peak levels and such links must therefore be carefully qualified
before you use them to carry the peak-controlled output of the 5750 to the transmitter. For example, the MPEG
Layer 2 algorithm can increase peak levels up to 4 dB at 160kB / sec by adding large amounts of quantization noise
to the signal. While the desired program material may psychoacoustically mask this noise, it is nevertheless large
enough to affect peak levels severely. For any lossy compression system the higher the data rate, the less the peak
levels will be corrupted by added noise, so use the highest data rate practical in your system.
It is practical (though not ideal) to use lossy data reduction to pass unprocessed audio to the 5750's input. The data
rate should be at least of "contribution quality"— the higher, the better. If any part of the studio chain is analog, we
recommend using at least 20-bit A/D conversion before encoding.
Because the 5750 uses multiband limiting, it can dynamically change the frequency response of the channel. This
can violate the psychoacoustic masking assumptions made in designing the lossy data reduction algorithm.
Therefore, you need to leave "headroom" in the algorithm so that the 5750's multiband processing will not unmask
quantization noise. This is also true of any lossy data reduction applied in the studio (such as hard disk digital delivery
systems).
For MPEG Layer 2 encoding, we recommend 384kB / second or higher.
Some links may use straightforward PCM (pulse-code modulation) without lossy data reduction. If you connect to
these through an AES3 digital interface, these can be very transparent provided they do not truncate the digital
words produced by the devices driving their inputs. Because the 5750's output is tightly band-limited to 16.5kHz, it
can be passed without significant overshoot by equally well by any link with 44.1kHz or higher sample frequency.
Currently available sample rate converters use phase-linear filters (which have constant group delay at all
frequencies). If they do not remove spectral energy from the original signal, the sample rate conversion, whether
upward or downward, will not add overshoot to the signal. This is not true of systems that are not strictly band-
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