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The Secret Life of HDMI – Part 5

Wed 23 February 2011 | Technical articles | Back to article list

In our last article we looked at the basics of surround sound formats. Understanding the various formats can be a confusing as there are many of them and their names can sometimes be very similar to each other!

This week we’ll take a brief look at how different versions of HDMI can be used to carry different types of surround sound. But first a simple fact; HDMI is a digital interface and can only carry digital signals. Analogue signals have their own interfaces (like the common RCA/Phono connector).

Back to Stereo
Stereo (two channel left & right audio) has been with us for decades (in analogue form) and at least thirty years as digital. Since v1.0 HDMI has always been able to carry stereo digital audio at up to 24bit resolution and 48KHz sampling frequency. While this may seem a bit restricted for home cinema use (where more channels are needed) we should remember that a stereo audio stream could also carry Dolby Pro-Logic information for basic surround sound audio. In fact many films and most TV shows only have a Pro-Logic option for surround.

Bitstream Digital 5.1/7.1
As we learnt from in our last post, the majority of discreet digital surround sound tracks are either 5.1 or 6.1 in either Dolby Digital or DTS formats. Both of these use “lossy” compression to squeeze more audio into the same space while throwing away information we (arguably) cannot hear.

In their raw form both Dolby Digital and DTS exists as “bitstreams”. The bitstream data format is different from the usual “SPDIF” format we see on the output of our CD players and consists of all the channels packed up together, still in a compressed form.

Again HDMI v1.0 and up can carry these simple bitstreams allowing basic connection of, say, a DVD player and receiver.

Uncompressed Data
From v1.1 HDMI also allowed the transport of multi channel digital audio in its uncompressed form (up to 8 channels of 24bit 192KHz audio). This ability uses much greater bandwidth but allows decompression from bitstream to multichannel in the source component (e.g. DVD player), which can then be passed to a processor over the same HDMI cable as video.

At the launch of HDMI v1.2 the expected use for this facility was by DVD-Audio players. Using HDMI they could send their higher quality PCM audio directly to an AV amplifier for amplification. Sadly the demise of DVD-Audio as a format has left this facility underused.

1 Bit Audio
The technical arguments for single bit audio over PCM are beyond the subject of this post. However, for audiophiles the discussion presented itself, at least in part, with the competition between DVD-Audio and SACD formats. While DVD-Audio offered conventional PCM (albeit in much higher resolution than CD) while SACD used single bit, high sampling frequency audio.

Single bit audio presents its own transport problems and these were addressed in v1.2 of the HDMI format, which added the possibility to transport single bit audio over that connection. Sadly both formats failed to find wide acceptance and remain a sideline to the main digital audio story.

High Data Bitstreams
The emergence of the BluRay disc brought new surround formats from Dolby and DTS in the shape of higher quality compressed or high-resolution uncompressed soundtracks. Again, this needed extra bandwidth to transport.

HDMI again had an answer in v1.3. The extra bandwidth offered by v1.3 allowed the connection of these new soundtracks from source to processor along the HDMI connection and a noticeable improvement in potential sound quality was then possible.

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