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Commentary: APT Has Its Own Take on Surround

In recent years, 5.1 surround sound has become the accepted mode for audio in the film, television and music industries, as opposed to the simple, two-channel stereo approach. Consumer demand has largely driven this increase in audio channels, with most households now owning a DVD player complete with a 5.1 speaker configuration.

In recent years, 5.1 surround sound has become the accepted mode for audio in the film, television and music industries, as opposed to the simple, two-channel stereo approach. Consumer demand has largely driven this increase in audio channels, with most households now owning a DVD player complete with a 5.1 speaker configuration.

Add to this, the HDTV audience, cinema-goers and the young “gamers” who grew up with the surround sound experience, and we realize there is now a level of consumer expectation that has to be met by content providers.

The simple fact is that the radio broadcast industry needs to provide 5.1 multi-channel audio content. Radio broadcasters are in a highly pressured market, competing against information and entertainment mediums such as TV, Internet, DVD, MP3 players and iPods. An old two-channel program simply is not going to meet the demands of their target audience.

It may be argued that a stereo signal is abnormal. Although we have two ears, the brain has the ability to interpolate many signals simultaneously, which creates a multi-dimensional image. In comparison, a two-channel signal will be both flat and unnatural.

Standards wars

Digital radio has made the incremental step of improving quality by increasing audio bandwidth and adding ancillary services, but this is primarily still focused on delivering two-channel content. To make a significant step, the next generation of digital radio services needs to offer the surround sound experience.

Several 5.1-for-radio pilot projects have been completed and the war of standards is raging for a suitable transmission protocol. Although several parties are involved, the two primary participants are Dolby and Digital Theatre Systems, with Microsoft starting to flex its powers.

The manufacturers of digital receivers probably will take the smart, well-trodden route of spinning-in both solutions. As such, in principle, listeners will be able to get 5.1 content into their homes through set-top boxes and cars.

Now comes the bigger challenge: How does the broadcaster move live content from remote locations such as music and sporting venues, through to their studios and then out to their transmitter sites?

One solution is to run linear. A 24-bit, 48 kHz sampled program requires a data rate of almost 6 megabits per second. For most broadcasters, such an option is cost-prohibitive and will kill any contribution and distribution projects at birth.

This brings us to our old friend compression and the balancing act between low-bit-rate algorithms, using perceptual coders, and higher bit rates, which use Adaptive Differential Pulse Code Modulation principles, or ADPCM.

Perceptual coders certainly will reduce the network costs but add substantial latency and run the risk of destroying the phase relationship between the individual channels.

Given the loss of stereo separation caused by perceptual coders in stereo signals, this is probably a given. It is also worth noting that the final transmission algorithm, whether DTS or Dolby, will be highly bit-rate reduced, and all efforts toward conserving content should be made prior to the final transmission.

5.1 pilot projects

An ADPCM-based algorithm will offer a much lower coding delay and will retain the phasing between the channels.

Enhanced apt-X from APT generally is considered by users and third-party licensees of the algorithm in the broadcast and post-production industries to be non-destructive in nature and offers end-to-end system latency of fewer than 5 milliseconds, making it a suitable solution for 5.1 contribution and distribution.

Another option is Dolby E, which often is touted as a solution but suffers from a few fundamental problems including a latency of over 60 milliseconds, bit polling between channels, an inability to process individual channels and limited word resolution.

As previously stated there have been a few pioneering broadcasters who have conducted on-air 5.1 pilot projects. Interestingly, these projects are going on in parallel on both sides of the Atlantic, i.e. ORF in Austria and NPR in the United States. Unsurprisingly, the Europeans and the Americans have explored two fundamentally different techniques, but key to both was the use of Enhanced apt-X.

ORF, under the guidance of Karl Petermichl (and closely observed by his fellow EBU members), chose Enhanced apt-X wrapped up in the WorldNet SkyLink. This unit is a codec with eight discrete channels (5.1 and a stereo pair) and it uses an Ethernet port to present the compressed data to the outside world.

The WorldNet SkyLink units were used for projects that included “Night of the Long Radio” and ORF’s New Year’s Eve broadcast. The discrete-channel approach enabled ORF to process individual channels and keep to a minimum the amount of hardware used in the broadcast chain. Data capacity was provided by Austria Telecom, which supplied a 2 Mbps ADSL circuit.

It is worth noting that German public broadcasters Westdeutscher Rundfunk in North Rhine – Westphalia and Bayerischer Rundfunk in Bavaria also are using Enhanced apt-X for 5.1 and are using an E1 interface. For these projects the broadcasters used the WorldNet Oslo. The actual transport medium, i.e. Synchronous or IP, would appear to be a decision based on what service the local Telco provides.

Enhanced apt-X

In the United States, NPR and KUVO(FM) have aired two pilot broadcasts. NPR and KUVO aired the “Toast of the Nation” New Year’s Eve event; KUVO broadcast a Diane Reeves concert. KUVO Chief Engineer Mike Pappas used a fundamentally different approach for moving surround sound content.

At the Reeves concert site Pappas down-mixed the 5.1 channels to two channels using the Neural Audio 5225 system. He then transported the two channels using ISDN codecs. The codec, the WorldNet Tokyo, bonds together 4 x ISDN lines to create a 512 kbps data pipe and uses Enhanced apt-X at 24-bit word resolution, 48 kHz sampling frequency.

As Enhanced apt-X is a non-destructive compression algorithm, this removed the incidence of any artifacts affecting the downmix, something that an MPEG algorithm could not achieve. At the receiving end, NPR reconstituted the two channels back up to a 5.1 mix.

The results of pilot projects undertaken by these pioneering broadcasters are likely to shape and influence the decisions taken when large-scale deployment of 5.1 multi-channel broadcasting begins in earnest. While ORF and NPR adopted two fundamentally alternative approaches using different equipment and transport mediums (and APT does not claim that one approach is superior to another), the constant in both scenarios was the choice of coding algorithm.

All parties agreed that the APT products, and the Enhanced apt-X algorithm, provided the right balance of network efficiency, latency and flawless audio quality.

RW welcomes other points of view.