Total Harmonic Distortion (THD)

Before I dive into discussing Total Harmonic Distortion (THD), let me take time to clarify a few terms related to it in the context of audio systems.
In Audio Systems, the behavior of linear and nonlinear audio signals can be adequately represented by a sinusoid. Nonlinear signals can always be decomposed to multiple sinusoids using an FFT technique (Fast Fourier Transform). When a sinusoidal signal is impressed upon a linear system, the output is also a sinusoidal signal, which in some cases might be phase or amplitude shifted. The scenario is quite different for a nonlinear system. In this case, the output signal is composed several sinusoids of smaller amplitudes or magnitudes superimposed on the original signal as can be seen in the next diagram. If you are outputting such a signal to a driver (speaker), it manifests elements of distorted percussion or noise which becomes significant with increase in the power of  the unwanted sinusoids. This phenomenon is known as harmonic distortion.

Harmonic Distortion is thus a form of nonlinearity in which unwanted signals harmonically related to the the input signal are added to it. These unwanted harmonic signals always manifest at integral frequency multiples of the fundamental frequency (input signal), i.e at 2x the original signal, 3x, 4x, 5x, etc, and none at say, 1.5x the original, or any fractional multiplier. 

If you  were to sum up the output power of all the resulting harmonic signals and express them as a ratio to the output power of the fundamental component, then you will have determined the Total Harmonic Distortion for that particular audio system. This is however never an easy feat to achieve for it is not practical and critical to measure all the harmonics for the entire audio signal range, 20Hz - 20kHz. Instead, manufacturers tend to ignore harmonic components at barely audible frequencies from 10kHz - 20kHz.  Very often, THD is specified at 1kHz. A low THD value implies a minimally distorted resulting output audio signal and consequently a good audio system or Amp.

 Some known facts about THD in audio systems

Accurately measuring individual harmonics for the entire signal range is a very laborious task and is achieved by sweeping and plotting results on a spectrum analyzer to obtain the level of each harmonic. Manufacturers, do commonly avoid this THD test and instead opt for the simpler THD + Noise (THD+N), though in gadget sheets you  see plain THD. In the THD+N test, a specified number of harmonic amplitudes are measured under a specific test signal frequency, level and gain settings on the tested unit.

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DTS Audio Codecs

DTS originally by Digital Theater Systems, just like Dolby Digital is a multichannel audio coding technology converts stereo sound sources to 5.1-channels but implemented  differently. Today, DTS and Dolby Digital are both implemented in DVD and home theater applications. In addition to the standard 5.1-channel DTS Surround codec, the company has several other technologies in its product range designed to compete with similar systems from Dolby Labs. These conceptually extend the core DTS Coherent Acoustics codec data stream to add more channels. The core stream is compatible with DTS decoders which do not support the extension(s) that provide additional data required to implement additional functionality.
In the consumer applications DTS is said to deliver a perceptually transparent performance when encoded at its highest legal bit rate of 1509.75kbps. However, in practical applications neither Dolby Digital nor DTS is run at their nominal bit rates owing to bandwidth and storage space constraints. Dolby Digital also known as AC-3 is capped at 448kps while DTS at 754.5kbps.

DTS Express

DTS Express is a proven high quality, low bit-rate audio codec that has been widely used by major entertainment studios for Blu-ray secondary audio, BD Live, and other similar applications. DTS Express can be used with up to 5.1 channels and up to 48 kHz/24 bit. This technology also features constant bit rates from 48 kbps to 512 kbps and is fully compatible with hundreds of millions of consumer electronics devices with a DTS Digital Surround™ real-time transcoder. DTS Express also features the exclusive “Fit-To-Stream” technology for digital delivery tailored for consumers downloading entertainment content via the internet.

DTS Digital Surround

There are several variants of DTS Digital Surround™ and include the following;

• DTS Digital Surround: capable of delivering up to 5.1-channels of up 48 kHz/24-bit and at constant bit rates of up to 1.5 Mbps. It is compatible with hundreds of millions of DTS decoders in the world today. It forms the core of DTS-HD codecs, resulting in backward compatibility with legacy DTS playback devices.
• DTS Digital Surround 96/24: Has a sample rate of 96 kHz and bit-depth of 24 bits. It delivers superior frequency response, tonal accuracy, and dynamic range. It is the only surround audio format capable of delivering 96 kHz over an S/PDIF connection.
• DTS Digital Surround ES: This adds a discrete center surround channel to deliver an exciting 6.1 discreet sound channel experience. The additional channel increases sound localization and offers creative opportunities to the sound mixer, thus offering a more engaging surround sound experience to the consumer. DTS Digital Surround ES is the only compressed digital audio format that is capable of delivering 6.1 discrete channels of audio over S/PDIF.
• DTS Neo:6 is comparable to Dolby's Prologic IIx. It creates up to six full-bandwidth matrix channels of surround sound from any stereo audio source.  DTS Neo:6 generates a proper center channel, along with accurately directing audio to either a 5.1 or 6.1 setup, based on the destination speaker layout, which naturally follows the original presentation.
• DTS Neo:X can take stereo, 5.1, 6.1, or 7.1 source material and output up to 11.1 channels including front height and width channels. Unlike Dolby's Pro LogicIIz system, which only adds front height channels to the 7.1 configuration, Neo:X supports both front height and front wide channels. DTS Neo:X also supports 11.1 encoding through matrixing of front height and front wide channel information into the front and surround channels, respectively, of a 5.1 or 7.1 audio mix.

DTS-HD High Resolution Audio

DTS-HD High Resolution Audio is a high-resolution audio format, which provides a higher quality audio experience than standard definition surround sound. It has been adopted for Blu-ray Disc™. DTS-HD High Resolution Audio can deliver up to 7.1 discreet channels and 96kHz/24-bit. It provides constant bit rates from 2.0 to 6.0 Mbps. In fact in some applications it's used as an alternative to DTS-HD Master Audio™ because it uses up less space in comparison while delivering a high definition audio experience.

DTS-HD Master Audio (DTS-HD MA)

DTS-HD Master Audio allows up to 7.1 discreet channels at 96kHz or 5.1-channels at 192kHz and is backward compatible with legacy DTS decoders. DTS-HD Master Audio is the premium high definition format for all high definition applications. It reproduces the studio master recording bit-for-bit and utilizes variable bit-rate technology to deliver an uncompromised audio experience that brings movies, music and games to life. DTS-HD Master Audio has been adopted as an industry wide standard for for Blu-ray disc. It's non redundant structure enables for a more streamlined production. The savings in data and bandwidth enables content creators to increase video quality, add alternate languages, special features and other added value material. DTS-HD Master Audio is selected as an optional surround sound format for Blu-ray Disc and HD-DVD, where it has been limited to a maximum of 8 discrete channels. DTS-HD MA supports variable bit rates up to 24.5 Mbit/s on a Blu-ray Disc and up to 18.0 Mbit/s for HD DVD, with up to channels encoded at up to 192kHz or 8 channels encoded at 96kHz/24 bit.

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Dolby Audio Codecs

Dolby Digital audio compression technology was developed by Dolby laboratories for cinematic applications. Also referred to as AC-3, is an advanced audio encoding/decoding technology that efficiently delivers up to 5.1 discrete channels of rich, vibrant and enveloping surround sound for broadcasting, home theater, cinema, PC, online streaming, and video game programming.

The three front channels (Left, Center, and Right) provide crisp, clean dialogue and accurate placement of on-screen sounds, while the twin surround channels (Left Surround and Right Surround) create an immersive effect into the action for the audience. The Low-Frequency Effects (LFE) channel delivers deep, powerful bass effects that can be felt as well as heard. It delivers low-pitched sound in the range 3-120Hz to Subwoofers. This channel uses up only about one-tenth the bandwidth of each of the other channels. Dolby Digital offers a maximum bit rate of 640kbps. Both Blu-ray and HD-DVD are required to support Dolby Digital at its maximum bit rate. Because Dolby® Digital audio codec is so efficient; it has been widely adopted as a standard for DVD as well as Blu-ray Disc™. It has also been adopted as the preferred standard for Satellite SD/HDTV in US, Canada, Australia and UK.

Dolby Digital specifics

• 1.0 to 5.1, discrete
• 32–640 kbps, scalable
• DVD data rate, 5.1-channel audio: 384 or 448 kbps
• Blu-ray Disc data rate, 5.1-channel audio: 640 kbps
• Supports Dolby metadata
• Connections: S/PDIF, HDMI, IEEE 1394
• Capable of Mixing/streaming 

Dolby Digital Plus

Dolby Digital Plus is a versatile digital audio coding technology that extends the capabilities of Dolby Digital. Dolby Digital Plus delivers up to 7.1 channels of cinematic surround sound, with scalable sound delivery and a wide range of bit rates ensuring optimized sound quality and efficiency for the available bandwidth. With more channels, less compression, and higher bit rates, Dolby Digital Plus helps equipment manufacturers and content producers differentiate their products by delivering a sound experience that some reviewers have described as being “virtually transparent” to the original source.  

 

Dolby Digital Plus specifics

• 1.0 to 7.1 discrete channels
• 32 kbps–6 Mbps scalable data rate; typical range on HD optical discs is 768 kbps–1.5 Mbps
• Supports Dolby metadata
• Supported by  S/PDIF, HDMI, IEEE 1394 connections  
• Capable of streaming/mixing
• Backward Compatibility; S/PDIF to legacy A/V receivers (640 kbps data rate)

 

Dolby® Digital Surround EX™

Dolby® Digital Surround EX™ is created by introducing an additional rear surround audio channel to the 5.1 channel Dolby Digital. Adds a center rear channel to the Dolby Digital 5.1 system for increased cinema realism and creates a richer enveloping surround sound with sound effects panning from front to back. Supports 6.1-channel home audio playback.

Dolby® Pro Logic® II

Dolby® Pro Logic® II is an advanced matrixing technology that delivers convincing, realistic surround sound from stereo content. It delivers a natural, realistic and compelling five-channel surround sound from stereo program sources; namely TV broadcasts, movies, video games, and music. Dolby Pro Logic II technology is also used by broadcasters to encode their stereo signals specifically for Pro Logic II playback. This makes it possible for them to deliver specific surround and localization effects over stereo services that are more like discrete 5.1 transmissions, without compromising regular stereo and mono playback.

Dolby® Pro Logic® IIx

Dolby® Pro Logic® IIx is an extension of Dolby Pro Logic II matrixing technology that delivers enriched enveloping 6.1 and 7.1 surround sound from both stereo and 5.1 content. It enhances spatial depth and improves sound directionality. With this technology the listening area is perceptibly larger and provides soundfields tailored for movies, music and gaming.

Dolby® Pro Logic® IIx specifics 

• Ability to generate up to 7.1 channels from stereo and 5.1 sources
• Supported decoding: 
• Pro Logic mode  
• Virtual Compatible mode  
• Music mode  
• Movie mode  
• Matrix mode  
• Dolby Digital EX mode  
• Supported upmixed output configurations based on stereo sources: 
• 2 to 3 [Left, Right to Left, Center, and Right]
• 2 to 4 [Left, Right to Left, Right, Left surround, and Right surround]
• 2 to 5.1 [Left, Right to Left, Center, Right, Left surround, and Right surround] 
• 2 to 6.1 [Left, Right to Left, Center, Right, Left surround, Right surround and Center surround] 
• 2 to 7.1 [Left, Right to Left, Centre, Right, Left surround, Right surround, Left rear surround, and Right rear surround]
• Supported upmixed output configurations based on 5.1 sources with independent Ls and Rs channels: 
• 5.1 to 6.1 [Left surround, Right surround to Left surround, Right surround, and Center surround] 
• 5.1 to 7.1 [Left surround, Right surround to Left surround, Right surround, Left rear surround, and Right rear surround] 

Dolby® Pro Logic® IIz  

Dolby® Pro Logic® IIz adds front height channels to surround sound, creating a 7.1 or 9.1 playback system for home music, movies, and video games. Dolby Pro Logic IIz brings enhanced spatial effects, added depth, and an overall airiness to the listening experience. Essentially, it’s an extension of Dolby Pro Logic IIx by front height channels. It is possible to expand a 5.1 playback system to a 7.1, or a 7.1 system to a 9.1 while offering a near discreet listening experience. By adding front height channels, Dolby Pro Logic IIz creates a 7.1 or 9.1 playback system that enhances the spaciousness of music, movies, and video games, by introducing a vertical component to the horizontal soundfield of a conventional 5.1 or 7.1 surround system.  

 

Dolby® Pro Logic® IIz specifics  

The features listed above allow Dolby Pro Logic IIx to provide a true surround sound experience from a variety of sources. However, all the outputs derived by existing Dolby Pro Logic IIx decoders are located in a horizontal plane. The Dolby Pro Logic IIz extensions to Dolby Pro Logic IIx allow it to derive height channel outputs that significantly enhance the spatial experience. In addition, Dolby® Pro Logic® IIz supports the following;

• Ability to generate audio of up to 9.1 channels that include height channel outputs from stereo, 5.1, and 7.1 Back sources.
• Supported upmixed output configurations based on stereo sources:
• 2 to 7.1 Height [Left, Right to Left, Center, Right, Left surround, Right surround, Left height, and Right height]
• 2 to 9.1 [Left, Right to Left, Center, Right, Left surround, Right surround, Left rear surround, Right rear surround, Left height, and Right height]
• Supported upmixed output configurations based on 5.1 sources with independent Ls and Rs channels:
• 5.1 to 7.1 Height [Left surround, Right surround to Left surround, Right surround, Left height, and Right height]  
• 5.1 to 9.1 [Left surround, Right surround to Left surround, Right surround, Left rear surround, Right rear surround, Left height, and Right height] 

Dolby TrueHD 

Dolby TrueHD offers playback that is as close as it gets to the sound captured at a live concert. Its powerful sound is bit-for-bit identical to what is created in the recording studio or mixing stage. Dolby TrueHD brings a spacious sound stage, highly accurate effects, and unsurpassed dynamic range to home theaters, delivering a compelling “you are there” surround sound experience. It’s 100 percent lossless, bit-for-bit identical to the original studio recording. It conserves Disc space through the 4:1 compression to limit bandwidth usage. For playback through a home theater system, Dolby TrueHD is transported via HDMI® (High Definition Multimedia Interface) version 1.3 or later, a single-cable digital connection for audio and video. Dolby TrueHD content may also be introduced to a playback system via Ethernet or USB connections.  

Dolby TrueHD features 

• Dialogue normalization maintains consistent volume levels when switching between Dolby TrueHD and Dolby Digital or Dolby Digital Plus programming. 
• Dynamic range control enables reduced peak volume levels without impacting overall sound quality. It’s ideal for late-night viewing. 

• Downmixing allows the content creator to enable stereo playback from the original multichannel source or to provide an independent stereo “artist’s mix” that is incorporated into the Dolby TrueHD bitstream.
• Three presentations, one bitstream: A single Dolby TrueHD stream can contain up to three separate mixes (presentations): a 7.1-channel surround mix, a 5.1-channel surround mix, and a two-channel “artist’s” stereo mix. This gives content creators the ability to tailor each mix for optimum listening on any playback system. 

• Advanced 96k upsampling: Movie soundtracks and TV shows are often recorded using a 48 kHz (48k) sampling frequency. The brickwall filters required for 48k analog-to-digital (A-to-D) conversion in the recording process introduce artifacts called preringing, heard as an unnatural hardness or edginess. To deliver clearer, more natural playback sound from this content, Dolby TrueHD encoding now includes a proven 96 kHz (96k) upsampling process developed by Meridian Audio. It significantly improves the sound from a Blu-ray Disc in any channel configuration. The process applies higher sample rates and increased playback quality through an advanced apodizing filter that masks undesirable digital artifacts in 48k material. The upsampling takes place before Dolby TrueHD encoding.  By applying the upsampling in software, Dolby TrueHD creates content that’s compatible with all Blu-ray™ players and all A/V receivers with 96k digital-to-analog converters (DACs). It does not add any computing burden in playback. Although the upsampling increases file size and data rates, the optional Dolby Mapping (noise-shaping) feature, specifically designed for Dolby TrueHD, minimizes or eliminates these increases. Content producers can employ the upsampling feature at their discretion. The process is designed to improve 48k audio files and is not needed for native 96k material. 

• Dolby TrueHD can deliver up to 16 channels of pristine audio, so it is ready for any future HD streamed formats. On Blu-ray Disc today, it delivers up to eight full-range channels of 24-bit/96 kHz audio and supports up to 24-bit/192 kHz for stereo and 5.1 content. 

• Leverages interchannel correlations to reduce data rates for lossless matrixing and more efficient downmixing.
• Employs decorrelation to take advantage of the predictive nature of audio. 

• Allows fewer bits to be assigned for commonly occurring values within the data stream for a more efficient coding process using Huffman (entropy) coding.

• Smooths the encoded data rate with managed FIFO buffering throughout audio data transmission.

 

Dolby TrueHD Specifics 

Dolby TrueHD supports;

• 44.1 kHz to 192 kHz sampling frequency.
• Variable bit rate; CBR rendering for transport over HDMI
• Word length up to 24 bits.
• Up to eight channels of 24-bit/96 kHz Blu-ray audio; designed to be expandable should the Blu-ray Disc standard adopt additional channels.
• Two- to six-channel of up to 24-bit/192 kHz. 
• Maximum bit rate of up to 18 Mbps on Blu-ray Disc.

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