Orban OPTIMOD-FM 8700, Operating Manual

Page: 46 / 390

1-16
INTRODUCTION ORBAN MODEL 8700i
case) to approximately 60dB (see Figure 2-3 on page 2-10). This separation is com-
fortably above the separation (approximately 20dB) that starts to cause perceptible
changes in the stereo image.
2
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 unpro-
cessed audio for application to the 8700i’s input, (2) they can pass the 8700i’s peak-
controlled analog or digital left and right audio outputs, or (3) they can pass the
8700i’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 re-
sponse 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 in-
puts 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 los-
sy 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 al-
most invariably distort peak levels and such links must therefore be carefully quali-
fied before you use them to carry the peak-controlled output of the 8700i 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
2
Julie M. Adkins and Robert D. Sorkin: “Effect of Channel Separation on Earphone-
Presented Tones, Noise, and Stereophonic Material,” J. Audio Engineering Society ,
vol. 33 pp. 234-239, 1985.
Subjects listened to 500-Hz tones, broadband noise, and stereophonic program ma-
terial through earphones and adjusted the channel separation, via a manual control,
until the degradation of the spatial effect became detectable. Mean channel separa-
tions ranged from 10 to 15.9 dB for the musical selections employed and from 13.7
to 16.8 dB for the noise and tonal stimuli. The results are discussed in terms of exist-
ing data on detectable stereo separation and on the discrimination of interaural
time differences. [Abstract ©Audio Engineering Society, Inc.]