Tag : Audio

AAC

Advanced Audio Coding is a digital audio coding system that delivers lossy compression offering about 25 percent efficiency improvement over popular MP3 coding at similar bit rates. However this performance was then topped by aacPlus, also known as High Efficiency AAC (HE-AAC). It is included in MPEG-4 and delivers ‘CD quality’ stereo at 48 kb/s and 5.1 surround sound at 128 kb/s. HE-AAC is also adopted in DAB+ and Digital Radio Mondiale as well as DVB-H and ATSC-M/H mobile applications.

Auditory masking

The psycho-acoustic phenomenon of human hearing where what can be heard is affected by the components of the sound. For example, a loud sound will mask a soft sound close to it in frequency. Audio compression systems such as Dolby Digital and MP3 audio make use of auditory masking as their basis and only code what can be heard by the human ear.

See also: Dolby Digital, MP3

AVB

Audio Video Bridging is, as of 2012, the Time-Sensitive Networking Task Group which aims to provide specifications to allow time-synchronized, low latency streaming services through IEEE 802 networks.

See also: Ethernet

BWF

Broadcast WAV File, a widely used audio file format based on Microsoft’s WAV. It can carry PCM or MPEG encoded audio and adds the metadata, such as a description, originator, date and coding history, needed for interchange between broadcasters.

See also: WAV

 

Compression (audio)

Reduction of bandwidth or data rate for audio. Many digital schemes are in use, all of which make use of the way the ear hears (e.g. that a loud sound will tend to mask a quieter one) to reduce the information sent. Generally this is of benefit in areas where bandwidth or storage is limited, such as in delivery systems to the home, hand-held players, etc. Generally the terms ‘coding’ or ‘codec’ refer to a compression scheme, such as MP3.

See also: Auditory masking, Dolby Digital

Decibel (dB)

Units of measurement expressing ratios of power that use logarithmic scales to give results related to human aural or visual perception. Many different attributes are given to a reference point termed 0 dB, for example a standard level of sound or power with subsequent measurements then being relative to that reference. Many performance levels are quoted in dB, for example signal to noise ratio (S/N).

Decibels are given by the expression:

10 log10 P1/P2

where power levels 1 and 2 could be audio, video or any other appropriate values.

Discrete 5.1 Audio

Often referred to as ‘5.1’, this reproduces six separate (discrete) channels – Left, Center, Right, Left Rear, Right Rear, and sub-woofer (the .1). All the five main channels have full frequency response which, together with a separate low-frequency sub-woofer, create a three-dimensional effect. Only one sub-woofer is required because the human ear is not directional at low frequencies. Discrete 7.1 Audio is similar but includes more speakers.

Discrete 5.1 audio is made available with many HD television broadcasts and is specified on HD DVD and BD media.

See also: Dolby Digital

Dolby

Dolby Digital (DD/AC-3) is a digital audio compression system that uses auditory masking for compression. It works with from 1 to 5.1 channels of audio and can carry Dolby Surround coded two-channel material. It applies audio masking over all channels and dynamically allocates bandwidth from a ‘common pool’. Dolby Digital is a constant bit rate system supporting from 64 kb/s to 640 kb/s rates; typically 64 kb/s mono, 192 kb/s two-channel, 320 kb/s 35 mm Cinema 5.1, 384 kb/s Laserdisc/DVD 5.1 and DVD 448 kb/s 5.1.

DVD players and ATSC receivers with Dolby Digital capability can provide a backward-compatible mix-down by extracting the five main channels and coding them into analog Dolby Surround for Pro Logic playback.

Dolby Digital Plus offers more, better quality, channels and supports data rates up to 6 Mb/s. is backwards compatible with Dolby Digital players and is offered as 7.1 channels on HD DVD and Blu-ray with data rates up to 3 and 1.7 Mb/s respectively.

Dolby E is an audio compression scheme which can encode/decode up to eight channels plus metadata – typically 5.1 mix (six channels) and Rt/Lt (Right Total/Left Total surround) or stereo two-channel mix, etc – onto two AES/EBU bitstreams at 1.92 Mb/s (20-bit audio at 48 kHz). Thus video recorders, typically with four channels, can support the greater channel requirements of DVD and some DTV systems (e.g. ATSC). With audio frames matching video frames, Dolby E is a professional distribution coding system for broadcast and post production which maintains quality up to 10 code/recode cycles.

Dolby E is widely used in HD production to carry 5.1 sound. As it is locked to video frames it has to be decoded and re-coded to work with a frame-rate conversion process.

Dolby Surround (a.k.a. Dolby Stereo, Dolby 4:2:4 Matrix) offers analog coding of four audio channels – Left, Center, Right, Surround (LCRS), into two channels referred to as Right Total and Left Total (Rt, Lt). On playback, a Dolby Surround Pro Logic decoder converts the two channels to LCRS and, optionally, a sub-woofer channel. The Pro Logic circuits steer the audio and increase channel separation. The Dolby Surround system, originally developed for the cinema, is a method of getting more audio channels but suffers from poor channel separation, a mono limited bandwidth surround channel and other limitations. A Dolby Surround track can be carried by analog audio or linear PCM, Dolby Digital and MPEG compression systems.

Dolby TrueHD is a lossless compression system designed for high-definition disk-based media and claims to be bit-for-bit identical to the studio master. Running up to 18 Mb/s up to eight 24-bit/96 kHz channels are supported on HD DVD and Blu-ray Disc standards, and is expected to feature in future A/V receivers and downloadable media. It can connect over HDMI.

See also: Auditory masking, ATSC, Discrete 5.1

Website: www.dolby.com

Embedded audio

Audio that is carried within a video stream such as SDI – so simplifying cabling and routing. The standard SMPTE 272M allows embedding up to four groups each of four mono audio channels in SD-SDI. For HD the standard is SMPTE 299M. Both can carry up to 16 channels of audio (8 pairs). SMPTE 299-2:2010 extends audio capacity over 3G SDI to 32 channels.

48 kHz synchronous audio sampling is pretty well universal in TV but the standard also includes 44.1 and 32 kHz synchronous and asynchronous sampling. ‘Synchronous’ means that the audio sampling clock is locked to the associated video. For example in SD this means 1920 samples per frame in 576/50I, or 8008 samples per five frames in 480/59.94I. Up to 24-bit samples are allowed but mostly only up to 20 are currently used.

48 kHz sampling means an average of just over three samples per line, so three samples per channel are sent on most lines and four occasionally – the pattern is not specified in the standard. Four channels are packed into an Ancillary Data Packet and sent once per line (hence a total of 4 x 3 = 12 or 4 x 4 = 16 audio samples per packet per line).

See also: 1000/1001

MADI

Multichannel Audio Digital Interface, widely used among audio professionals, defines the data format and electrical characteristics of an interface carrying multiple digital audio channels, as in the Audio Engineering Society’s AES10-2008. It is popular for its large channel capacity: 28, 56, or 64 channels at up to 96 kHz, 24 bits per channel, and up to 3000m connections over optical fiber (or 100m over coax).

MP3

A high-performance, perceptual audio compression coding scheme which exploits the properties of the human ear and brain while trying to maintain perceived sound quality. MPEG-1 and 2 define a family of three audio coding systems of increasing complexity and performance – Layer-1, Layer-2 and Layer-3. MP3 is shorthand for Layer-3 coding. MPEG defines the bitstream and the decoder but, to allow for future improvements, not an encoder. MP3 is claimed to achieve ‘CD quality’ at 128-112 kb/s – a compression of between 10 and 12:1. Not all listeners agree with that.

See also: Auditory masking

Website: www.mp3-tech.org

Scrub (audio)

Replay of audio tracks at a speed and pitch corresponding to jog speed – as heard with analog audio tape ‘scrubbing’ backwards and forwards past an audio replay head. This feature, which is natural for analog fixed-head recorders, may be provided on a digital system recording on disks to help set up cues.

TrueHD

Dolby’s lossless audio technology developed for high-definition disk-based media including Blu-ray Disc. It includes ‘bit-for-bit’ lossless coding up to 18 Mb/s and support for up to eight channels of 24-bit, 96 kHz audio. It is supported by HDMI.

Website: www.dolby.com

WAV (.WAV)

An audio file format developed by Microsoft that carries audio that can be coded in many different formats. Metadata in WAV files describes the coding used. To play a WAV file requires the appropriate decoder to be supported by the playing device.

Website: www.microsoft.com