Published on 3rd August, 2015 by Cable Chick

Digital Audio Explained

Digital Audio Explained


Digital Audio Explained

Find out what Digital Audio does in your home theatre, and discover what marketing jargon actually matters!

What is Digital Audio?

1993 called. It sounded all distorted!
Remember Me?

For the purposes of this Article, we'll be focusing on Blu-ray Disc audio formats and standards.

Unless your home entertainment system still exclusively consists of a cassette deck and a turntable, you're already using Digital Audio in your home theatre. CDs, DVDs and Blu-ray Discs all use Digital Audio (in a variety of formats and codecs), and it is most commonly carried by HDMI, DisplayPort, Optical and/or Digital Coaxial cables.

Digital Audio is a method of storing sound data in such a way that it can be played back or reproduced without any loss of quality. At its best, Digital Audio on a Blu-ray is identical to the audio mixed by the movie studio for cinemas. At its worst, Digital Audio can sound like something taped off the radio using a TalkBoy.

Analogue Audio vs Digital Audio

Analogue audio recordings, like those on magnetic tape or vinyl, are derived from a continuous electrical signal. This constant voltage fluctuation directly correlates to speaker membrane movement, and in turn, sound waves. Analogue audio is subject to noise and distortion at every step of the mastering and playback process, which has made it more difficult to work with accurately compared to Digital solutions.

While all audio is captured (at the microphone) and performed (at the loudspeaker) in analogue, it can be stored digitally for multi-track mixing, easy distribution and accurate reproduction.

Digital Audio is encoded in ones and zeroes so that it can be decoded back to Analogue in the exact same manner every time. Whether you're working with 2 Channel stereo or 7.1 Channel Surround Sound, everything we do in today's home theatre is delivered in a Digital format and converted back to electrical power to be sent to the speakers.

Recording to Digital

The accuracy of every Digital Audio recording is dictated by its sample rate and its bit depth. At our absolute best, the majority of humans have a hearing range between 20Hz and 20,000Hz (20kHz), and is often a few thousand Hz lower for most adults.

This spectrum is the reason CDs were mastered at a 44.1kHz sample rate, and that the minimum sample rate for Blu-ray audio is 48kHz - the Nyquist Theorem states you must record at twice the sample rate of the highest frequency you want to reproduce.

Why 48kHz?
48,000Hz is neatly divisible by video frame rates, like 60fps, 30fps and 24fps. This makes timing and synchronisation easier. It is also arguably the highest sample rate we really need, offering a 24Khz top end that is outside the audible range.

In the following diagram, the original analogue audio is represented by the smooth wavy line. The sample rate is the number of times per second the wave form is looked at, and the bit depth is the number of possible values that can be recorded - the nearest intersections are represented by the red dots, and the black line is the digital copy of the analogue waveform.

Digital Audio - Low Resolution Example

As you can see, with a low sampling rate and low bit depth, frequencies can't be captured accurately. Playback of this digital copy won't sound anything like the original.

In this next diagram, we've increased both the sample rate and bit depth, giving us a better reproduction of the original sound (especially the low frequencies on the right side).

Digital Audio - High Resolution

When you consider that the current standard sample rate is nearly 50,000 times per second, and the two most common bit depths of 16 and 24 offer 65,536 and 16,777,216 data-points respectively, the original audio waveform can be reproduced with enough accuracy to make it objectively identical and demonstrably indistinguishable from analogue.

If a 48kHz 24bit resolution was used in a graph as above, the digital line would appear just as smooth as the analogue one.

Decoding Digital back to Analogue

Digital Audio decoding can happen at a couple of steps along the way from Blu-ray Disc to speaker. Some media players and games consoles will decode compressed audio themselves, and output it as multi-channel LPCM, or they may be configured to bit-stream the compressed Dolby or DTS format directly to your home theatre receiver. Whether this step occurs before your amplifier or not isn't terribly important, as the signal is still digital at this point. It is only when the digital signal is decoded into analogue that EQ profiles, SNR, THD+N and DAC quality (ie: the quality of the receiver/amplifier) come into play.

In any home theatre configuration which has multiple speakers a receiver, amplifier, sound-bar or similar hub device will handle all the analogue decoding. It is this decoder which does the job of converting the digital ones and zeroes back into voltage, which is then amplified and fed to the speakers. In surround-sound configurations, the decoder must process as many as 8 channels and deliver them synchronously to the correct speaker.

If you rely on your TV's built-in speakers to handle audio, the best you'll ever get is 2-channel stereo, which may be piped straight from the LPCM 2-channel track of a Blu-ray or DVD. The audio fidelity may also suffer as built-in speakers tend to sound pretty ordinary compared to free-standing speaker cabinets. In this example, the TV will be handling the analogue decoding, except of course if it has an SPDIF output, which is likely to also be stereo to match the track being played from the disc.

Audio Codecs and Formats

Dolby and DTS are the two leading companies behind cinema and home theatre audio. They set the standards for both lossless and lossy compression, and offer formats which support a wide range of sample rates and bit depths available over HDMI v1.3 or greater.

The table below shows what compression method is used and their highest sampling resolutions per number of channels.

  Method Max Bandwidth Max Bitrate Max Channels &
Max Sample Rate
Dolby True HD Lossless 18 mbit/s 24 7.1ch @ 96kHz or
5.1ch @ 192kHz
Dolby Digital Plus Lossy 4.7 mbit/s 24 7.1ch @ 48kHz
Dolby Digital Lossy 640 kbit/s 24 5.1ch @ 48kHz
DTS-HD Master Lossless 24.5 mbit/s 24 7.1ch @ 96kHz or
5.1ch @ 192kHz
DTS-HD High Res. Lossy 6 mbit/s 24 7.1ch @ 96kHz
DTS Digital Lossy 1.5 mbit/s 24 5.1ch @ 48kHz
LPCM Lossless 27.6 mbit/s 24 7.1ch @ 96kHz or
5.1ch @ 192kHz
Bitrate, Sample Rate and Channel count are all source media dependent

Not all formats are available on all Blu-ray discs, so always check the back of the case to see if your favourite is present at the level of quality you expect.

LPCM, or Linear Pulse-Code Modulation, is a Digital Audio format which is stored totally uncompressed. This takes up a lot of data and processing power, so even though it can accommodate up to 8 channels it's usually only used for a basic stereo track when on-disc.

While it is considered better to use a lossless audio format like Dolby True HD or DTS-HD Master Audio whenever possible, in most good quality consumer-level home theatre systems the average human will have trouble hearing a difference between lossy and lossless compression formats.

SPDIF (either Optical or Electrical) can carry either 2-channel uncompressed LPCM audio or compressed multi-channel Dolby Digital or DTS Digital, but not the lossless formats.

Expect to spend tens of thousands of dollars to make the most of studio-quality lossless audio formats at high sample rates, as the ultrasonic frequencies recorded at 192kHz are difficult to reproduce (and impossible to hear).

What is 5.2 or 7.2 Channel Sound?

It's just a case of adding a second subwoofer. If you like your bass, and your amp offers a second connection, you can run two subwoofers and upgrade any speaker system to .2.

What is 6.1 or 6.2 channel?
6.1 speaker configurations have an additional rear centre speaker for additional surround sound effects, plus one or two subwoofers as explained above.

What is Dolby Atmos?

Dolby Atmos may be the future of cinema and home theatre audio. By using as many as 64 speakers and 128 discrete audio tracks, Dolby Atmos is able to position audio within a room with remarkable precision.

In conjunction with 3D films or perhaps Head-Mounted VR Displays, Dolby Atmos has the ability to make our entertainment more immersive than ever before, with sounds moving around us in 3D space in a realistic manner. It could offer revolutionary advancement for games in the First Person, Simulator or Horror genres, too.

At the time of writing, Dolby Atmos is in its infancy, with only a few Blu-ray releases including an Atmos track, and even fewer consumer-level home theatre receivers capable of decoding its positional metadata. Purchasing and installing two dozen or more speakers in a home theatre may be impractical today, but it could become a new standard in the future for audiophiles and cinephiles.

The good news is that Dolby Atmos does not require a new version of HDMI, so cables and devices which don't decode audio to LPCM (or can be set to pass through the original encoded information) should pass Dolby Atmos through without any trouble, too. Gadgets like HDMI splitters and switches should be fine on Atmos-ready systems.

Converting between Digital and Analogue

No matter how fancy audio formats get, there will always be a call for regular stereo audio. Every Blu-ray has a stereo 'fallback' track for when it is played on a system with only two speakers or a decoder too old to understand surround sound formats.

This fallback stream can be easily converted to stereo Analogue so that it can become accesible to old amplifiers, earbuds or wireless headphones, etc. We have a couple of converters which can handle this task:

 

Likewise, we sometmes hold onto media players (like our trusty cassette deck) which only output analogue audio when we really need them to connect to a digital system. We also stock a converter which can convert Analogue stereo to Digital:

 

If your TV outputs TOSLINK Optical but your receiver can only accept Digital Coaxial, you're in luck. Converting between optical and electrical SPDIF formats is as simple as this product:

 

Still stuck in the past?

Digital Audio can be tough to wrap your head around. Fortunately it's mostly a case of plug and play, but if you have any questions, please contact us - we'll pass on our knowledge as best we can!