2017 NAB Introduction!
Optimod-FM 8700i: Processing for Digital Radio
Suitable for any digital transmission channel (such as DAB+ and HD Radio) that does not use pre-emphasis, the 8700iHD’s Digital Radio processing chain offers an ITU-R BS.1770-3 Loudness Meter and Loudness Controller for use in countries that enforce a BS.1770 loudness limit on digital radio broadcasts.
The 8700iHD implements “true peak” control in the HD processing chain by oversampling the HD peak limiter's sidechain at 256 kHz, which exceeds the recommendations in ITU-R BS.1770 for “true peak” metering. This allows the 8700i to prevent clipping in a playback device's analog signal path by predicting and controlling the analog peak level follow the playback device's reconstruction filter to an accuracy of better than 0.5 dB. For typical program material, accuracy is 0.2 dB.
Without true peak control, analog clipping can occur even if all peak values of the digital samples are below 0 dBFS. This phenomenon has also been termed “0 dBFS+.” Thanks to true peak control, sample rate conversion, unless it removes high frequency program energy or introduces group delay distortion, cannot cause sample peaks to increase more than 0.5 dB.
HD Radio: The HD Radio system generates a digital carrier that shares a given station’s allocated bandwidth with the analog FM carrier. The receiver crossfades 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 crossfades 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. 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 bitrate 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 8700i 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 the 8700i’s improved MX peak limiter, overshoot compensation, stereo encoding, and composite limiting using Orban’s patented “Half-Cosine Interpolation” algorithm. The peak limiting is anti-aliased and uses sample-rates as high as 512 kHz.
Meanwhile, the HD output receives low-IM look-ahead peak limiting, which we improved in the 8700i to exploit some of the same new technology we designed for the FM analog processing chain. This look-ahead limiting is optimized to make the most of the limited bitrate 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 useful 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 bitrate 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 8700i’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. In addition to be adjustable via 8700i PC Remote software, the diversity delay can also be adjusted remotely via the 8700i’s control API or SNMP. This provides “hooks” that allow a compatible third-party HD monitor receiver to automatically achieve exact delay matching at the receiver by measuring the delay and correcting it as necessary.
The 8700i’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 bitrate 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.
Although the 8700i’s MX peak limiter technology has narrowed the gap, the 8700i’s digital output still sounds cleaner and more open than its FM output, particularly in the high frequencies — the analog channel is inevitably handicapped by the standard 50 and 75 microsecond pre-emphasis curves, which compromise its high frequency headroom. Using program material, we’ve measured as much a 9 dB difference in favor of the digital channel at high frequencies! Even after the processed passes through the codec, a significant amount of this audible superiority remains.
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 8700i’s AES3 outputs can be locked to either the 8700i’s sync reference input or to its AES3 input, the 8700i fully meets the requirements. Moreover, because of the 8700i’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 HD Radio exciter and to use the 8700i’s built-in stereo encoder and composite limiter to drive the analog FM exciter directly.