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Coder-decoder - this page is a stub

A CODEC is a device or computer program which encodes and decodes an analogue signal into a digital data stream and vice versa. The expression CODEC is a portmanteau 1 of coder/decoder. The coder part is typically used to convert sound or images from the analogue world to the digital domain, and is also known as a digitizer. In the case of speech, it is also known as a VOCODER. 2

  1. A combination of two (parts of) words into one new word.  Wikipedia
  2. VOCODER = voice-coder.

Audio CODECs
Modulation techniques
There are several techniques for digitising audio signals, for example:

  1. Pulse Code Modulation (PCM)
    PCM is a generic expression for digitizing an analog signal. A PCM stream is the nu­merical or digital representation of the analog signal. Sending PCM data typically requires twice the bandwidth of the original analog signal, but the quality is unsurpassed.  More

  2. Delta modulation (DM)
    This technique uses only 1 bit/sample, as it only registers the difference between the current sample and the previous one (1 = higher, 0 = lower). This means that the signal can only increment or decrement in single discrete steps. A higher sample rate will result in a closer approximation of the original sound wave and, hence, a better quality.  More

  3. Continuous Variable Slope Delta modulation (CVSD)
    This is an improved version of Delta Modulation (DM) that gives a better approximation of the original sound waves at lower bit rates. It has a typical sample rate between 8 and 16 kHz, which results in 8 to 16 kbps data. Examples of equipment that uses CVSD are the Philips Spendex 10, the Spendex 50 and the American KY-68.  More
Compression   Vocoders
Before speech can be encrypted, is has to be converted from the analogue to the digital domain, by means of a sampler, or digitizer, or vocoder. Generally speaking, a digital signal needs more bandwidth than its analogue equivalent (typically twice the bandwidth), but methods have been developed to reduce this, by analysing the properties of speech and sending these as parameters to the other end, where they are used to reconstruct, or synthesize, the original voice again.

This method is known as a Vocoder and is not always good enough to recognise a person's voice. The first vocoder, named VODER, was developed at Bell Labs in 1939. Its principle was first used during WWII on the transatlantic SIGSALY cryptographic telephone. A speech analyser/synthesizer is also known as a CODEC (coder-decoder). Here are some examples of speech digitisers:

  • Formant analysis
    This method is based on the way the human voice is generated in the vocal tract (throat and mouth) and is therefore only suitable for speech encoding. It analyses the properties of speech – such as pitch, voiced sounds, voiceless sounds (hiss) and formants – and stores these properties as parameters that can be used by the decoder to reconstruct (synthesize) an approximation of the original sound.

  • LPC - Linear Predictive Coding
    Early vocoder for narrow bandwidth connections. LPC-10 has a sampling rate of 8 kHz and a coding rate of 2.4 kbps. Developed by the NSA and used in the first generation secure terminal units (STU). Later versions have a coding rate of 800 bps.  More

  • RELP - Residual-Excited Linear Prediction
    Improved (but now obsolete) variant of LPC, and predecessor of CELP.  Wikipedia

  • CELP - Code-Excited Linear Prediction
    Improved variant of LPC and RELP that provides better speech quality at lower bitrates. CELP exists in many variants and is also used in MPEG-4 audio coding. It is the most widely used speech coding algorithm today.  Wikipedia

  • MELP - Mixed-Excitation Linear Prediction
    Medium quality vocoder, mainly used by the US Department of Defense for secure communication via satellites and and military radios. It has a sampling rate of 8 kHz and a coding rate of 2.4 kbps (2400 bps).  Wikipedia

  • MELPe - Enhanced Mixed-Excitation Linear Prediction
    High-quality low-bitrate enhanced version of MELP with a sampling rate of 8 kHz and a coding rate of 2400, 1200 or 600 bps.  Wikipedia

  • MRELP - Modified Residually-Excited Linear Prediction
    Improved variant of MELP and MELPe that produces better results at higher bitrates, for example at 9600 bps.
Music CODECs
  • PCM
  • CVBS
  • MP3
  • AAC
  • JPEG
  • MPEG
  • H.26x
  1. Wikipedia, Codec
    Visited June 2022.

  2. Wikipedia, Pulse-code modulation
    Visited June 2022.

  3. Wikipedia, Sampling (signal processing)
    Visited June 2022.
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Crypto Museum. Created: Thursday 09 June 2022. Last changed: Wednesday, 08 May 2024 - 13:55 CET.
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