Optimod-FM 8500S: Processing for HD Radio
The 8500S-HD's digital radio processing path offers an ITU-R BS.1770-3 compliant loudness controller and a 4x-oversampled, “true peak” limiter that prevents clipping from occurring at the output of the D/A converter in a consumer receiver or other playback device.
The HD Radio™ system generates a digital carrier that shares a given station's allocated bandwidth with the normal analog FM carrier. The receiver cross-fades between the analog and digital channels to minimize the effect of RF dropouts. This scheme requires audio processing for the two channels to be closely matched in texture to ensure that the receiver's cross-fades are seamless.
Optimum peak limiting for the two channels is very different. The analog channel requires state-of-the-art pre-emphasis limiting to achieve competitive loudness and minimize pre-emphasis-induced high frequency loss. This usually implies use of sophisticated distortion-cancelled clipping. The digital channel, on the other hand, has no pre-emphasis but is heavily bit-reduced with the HDC perceptual codec. The highest available rate is 96 kbps and many broadcasters are now multicasting with two 48 kbps channels.
This limited bit rate creates an entirely different set of requirements: the peak limiting must not use clipping because there is no bit budget available to encode clipping-induced distortion products. However, pre-emphasis limiting is unnecessary. The best technology for peak limiting the digital channel is look-ahead limiting, which can perform very clean peak reduction on flat channels, but which is unsuitable for pre-emphasized channels.
Optimod-FM 8500S is an excellent solution to his dilemma because its AGC and stereo enhancement are shared between the two channels, while equalization, multiband compression/limiting, and peak limiting are independent.
The analog FM path provides 8500-style distortion-cancelled clipping, overshoot compensation, stereo encoding, and composite limiting using Orban's patented “Half-Cosine Interpolation” algorithm. The limiting is anti-aliased and uses sample rates as high as 512 kHz. Meanwhile, the HD output receives low-IM look-ahead “true-peak” peak limiting. This look-ahead limiting is optimized to make the most of the limited bit-rate codecs used digital radio and netcasting channels. By eschewing any clipping, the HD processing prevents the codec from wasting precious bits encoding clipping distortion products, allowing the codec to use its entire bit budget to encode the desired program material.
For convenience, it is possible to couple the equalizer, HF enhancer, and multiband compressor/limiter setup controls of the two paths, allowing them to be matched easily. This is convenient in HD Radio installations where the station's goal is to minimize the audibility of analog/digital crossfades at the receiver. However, the ability to adjust the analog FM and digital radio paths separately allows users more latitude to fine-tune their audio. For example, a broadcaster who believes that selling the advantages of HD Radio to the public requires an obvious, audible difference between the analog FM and digital channels can generate this “wow!” factor. Dual-path processing also allows the digital media processing to be independently tuned to minimize artifacts in low bit-rate codecs, like those used in netcasting and HD Radio.
A built-in diversity delay of up to 16 seconds in the analog processing path simplifies installation in HD Radio facilities, freeing you from the need to use the delay line built into the HD Radio exciter. This allows you to use the 8500S's built-in stereo encoder and composite limiter to drive the analog FM transmitter, ensuring no-compromise analog-channel loudness. The diversity delay can be applied independently to any output emitting the analog-FM processing signal, so some outputs can be delayed while others are not.
The 8500S's 64 kHz base sample rate allows it to provide up to 20 kHz audio bandwidth at its HD output. The HD bandwidth is user-settable between 15 and 20 kHz to optimize the processing for the codec employed in the digital chain. Many low bit rate codecs operate better when fed 15 kHz audio because this enables them to use their available bit bandwidth most efficiently. This is particularly true for low rates, like 32 kbps. However, at higher sample rates, full 20 kHz bandwidth provides the same bandwidth as typical source material, so the user may prefer to use it for these higher rates.
Most HD Radio exciters require 44.1 kHz AES/EBU audio streams for both their analog-FM and digital inputs. The sample rates for both streams must be identical and must be locked to an external reference. This requires two AES/EBU outputs from a single-box processor. Because the output sample rate on either or both of the 8500S's AES3 outputs can be locked to either the 8500S's sync reference input or to its AES3 input, the 8500S fully meets the requirements. Moreover, because of the 8500S's built-in diversity delay on the analog-FM channel, it is possible (and usually desirable) to entirely bypass the analog-FM side of the iBiquity exciter and to use the 8500S's built-in stereo encoder and composite limiter to drive the analog FM exciter directly.