OPTIMOD-AM DIGITAL OPERATION
3-55
square wave, or triangle wave), the preset you have on air is saved and will be re-
stored when you switch back to O
PERATE.
The upper frequency of the sine and square waves is limited to 1 kHz to ensure that
their waveforms look correct while respecting the system’s intrinsic band limiting
caused by its digital, sample-data nature. The square wave’s maximum modulation
level is limited to 50% to protect transmitters.
Table 3-10: Test Modes shows the facilities available, which should be self-
explanatory
.
About the 9400’s Digital Radio Processing
The 9400 digital radio (HD AM) processing is designed to feed streaming, netcasting,
and digital radio channels. It is suitable for both the iBiquity® AM HD Radio system
(formerly known as “IBOC” — “In-Band On-Channel”) approved for use in the
United States and the Digital Radio Mondiale system, used in much of the rest of the
world.
The digital radio firmware implements a high-quality signal processing chain that
includes stereo equalization, five-band compression, and look-ahead limiting. Audio
bandwidth is 15 kHz, as per the iBiquity® HD AM system specification.
The digital radio processing shares the stereo enhancer and AGC with the
processing for the analog AM transmission.
The five-band limiter in the digital radio processing chain has its own set of user-
adjustable parameters that are independent of the parameters of the five-band lim-
iter in the AM analog transmission chain. This allows you to optimize the digital ra-
dio processing for the higher fidelity sound provided by the digital channel while
giving you the flexibility to process the analog channel as aggressively as you want.
The five-band limiter in the digital radio processing chain has its own set of output
mix controls, which provides further setup versatility — there is no need to compro-
mise the processing on the digital radio processing chain to accommodate the needs
of the analog channel.
An advanced-design look-ahead limiter controls the peak level of the digital radio
output. This look-ahead limiter is optimized to make the most of the limited bit-rate
codec (HDC at 36 kbps) used in the AM High Definition Radio system’s digital chan-
nel. By eschewing any clipping, the digital-channel output prevents the codec from
wasting precious bits encoding clipping distortion products, instead allowing the
codec to use its entire bit budget to encode the desired program material.
The digital radio output is designed to feed digital channels having no pre-
emphasis, which include almost all such channels. The only high-quality digital
channels using pre-emphasis of which we are aware are NICAM channels (which use
J.17 pre-emphasis) and some older CDs (which use EIAJ — 50µs/15µs shelving pre-
To Next Page
Loading...
Need help?
General
