Orban Optimod-PC 1101 Operating Manual Download

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OPTIMOD-PC INTRODUCTION

1-37

real-world receivers and will reduce the peak modulation of the transmissions if
it turns out that most receivers are clipping due to perceptual encoding over-
shoots. Our experience to date indicates that allowing 3dB headroom should
prevent audible overshoot-induced clipping in low bite-rate systems (e.g., 32
kbps streams), while 2dB is adequate for 128kbps and above. While some clip-
ping may still occur, it will have a very low duty cycle and will almost certainty be
inaudible.

In sound-for-picture applications, the goal is often to have the loudness of your
broadcast or stream be the same as the loudness of other broadcasts or streams so
that consumers do not have to readjust their volume controls when changing from
one program stream to another. Dolby digital broadcasts and streams contain “Dial-
norm” metadata to help achieve this goal. OPTIMOD-PC 1101 can be operated so
that it is cognizant of Dialnorm, making it easy to ensure correct loudness.

In these applications, it is common to set Dialnorm to allow a substantial amount of
headroom so that the 1101’s peak limiters seldom produce gain reduction. See

Setting Output Loudness in Dolby Digital transmissions

on page 2-41.

Monitoring on Loudspeakers and Headphones

In live operations, highly processed audio often causes a problem with

the DJ or

presenter’s headphones.

When its built-in inputs and outputs are used, the delay

through OPTIMOD-PC can be as much as 25 ms (or more, if the installer purposely
adds frame-makeup delay). This delay, although not usually audible as a distinct
echo, can cause bone conduction comb filtering of the DJ/presenters’ voices in their
ears. This is almost always very uncomfortable to them.

Delays through the computer’s internal WAVE audio can be much larger.
These delays are difficult or impossible for the user to control because
they are functions of the computer’s operating system. To minimize de-
lay, use OPTIMOD-PC’s hardware inputs and outputs.

•

All codecs add peak overshoot to the audio. This because they remove energy
that is psychoacoustically masked by the input audio. It is common for low bit
rate codecs to overshoot by 3 dB. To eliminate the possibility of clipping at the
decoder, it is wise to allow 3 dB of peak headroom at the encoder. In other
words, set OPTIMOD-PC’s output level control to –3 dBfs when driving an audio
encoder.

•

The 1101’s peak limiter is oversampled at 192 kHz. Because these digital samples
do not necessarily coincide with the peak levels of the analog signals they repre-
sent, even samples that are constrained to 0 dBfs may produce higher peak levels
after D/A conversion. The phenomenon is commonly known as “0 dBfs+”.

To prevent clipping and its resulting audible distortion, it is important for hard-
ware designers to allow headroom after the D/A converter to accommodate
these levels. Unfortunately, some hardware is poorly designed and does not pro-
vide the needed headroom. The only way to prevent clipping in this hardware is