World Broadcasting Unions Digital Radio User Manual - page 54

DIGITAL RADIO GUIDE

TERRESTRIAL TRANSMISSION SYSTEMS - HD RADIO

54

HD Radio system have also been shown to be multipath-free even in challenging

propagation conditions.

(3)

Frequency Response

It is difficult to evaluate the exact frequency response of a codec, because it will

change dynamically depending on the available bit rate and the difficulty of

encoding the instantaneous audio material.  A quick example can be demonstrated 

using a single carrier frequency sweep on any codecs.  Typically total frequency

response of 20 kHz is measured in such tests, even at 16 kbps.  On the other hand, 

encoding a rich stereo program on the same codec at 16 kbps, may result in a pure 

monophonic signal of less than 5 kHz.

Consequently, the codec has to be tailored to the program content being broadcast.

See Table 4.5 for MPEG Layer II implementation recommendations.

Table 4.5: Recommended MPEG II data and sampling rates 

for various program material

Voice program:

48 kbps . 24 KHz Sampling rate / Mono

Mono program with music:

80 kbps / 48 KHz Sampling rate / Mono

Oldies music:

112 kbps / 48 KHz / Joint Stereo

(4)

Audio Quality Ratings

The basic audio quality is defined by the codec used and the ruggedness of the

transmission channel.  The performance quality of different codecs when critical

material is encoded has been measured as shown in Figure 4.16.