멀티미디어 시스템 – Chepter 2. Multimedia fundamentals – Homework : Investigate practical analog digital conversion(modulation) technology

Investigate practical analog digital conversion(modulation) technology
Chapter 2. 과제 : 실용적인 아날로그 디지털 변환 (변조) 기술에 대하여 조사하여라.


Content

  1. PCM ( Pulse-code modulation )
  2. Various kinds of audio format

Pulse-code modulation


1. Investigate practical A/D technology

  1. PCM ( Pulse-code modulation )

    1. Wide range of dynamics, wide range of sound that can be recorded.
    2. Even if the collapse of the pulse waveform occurs, there is no adverse effect on the receiver when there is no obstacle to the identification of the presence of pulses.
    3. The necessary devices at the relay point of the transmission path are only the shape of the pulse, and there are many merits.
  2. DPCM ( Differential pulse-code modulation )

    1. This is a signal encoder that uses the baseline of PCM but adds some functionalities based on the prediction of the samples of the signal. The input can be an analog signal or a digital signal.
    2. A method of performing image compression using a property that adjacent image points have substantially the same value. Predictive encoding of an image is to predict the next smallest value using the surrounding image value while sequentially scanning each point of the image. Through this prediction, it is possible to express the actual image value even if the value is not stored, so that the capacity of the data can be reduced.
    3. If the input is a continuous-time analog signal, it needs to be sampled first so that a discrete-time signal is the input to the DPCM encoder.
      1. option1
        take the values of two consecutive samples; if they are analog samples, quantize them; calculate the difference between the first one and the next; the output is the difference, and it can be further entropy coded.
      2. option2
        instead of taking a difference relative to a previous input sample, take the difference relative to the output of a local model of the decoder process; in this option, the difference can be quantized, which allows a good way to incorporate a controlled loss in the encoding.  Applying one of these two processes, short-term redundancy (positive correlation of nearby values) of the signal is eliminated; compression ratios on the order of 2 to 4 can be achieved if differences are subsequently entropy coded, because the entropy of the difference signal is much smaller than that of the original discrete signal treated as independent samples.
  3. ADPCM ( Adaptive differential pulse-code modulation )

    1. This is a variant of differential pulse-code modulation (DPCM) that varies the size of the quantization step, to allow further reduction of the required data bandwidth for a given signal-to-noise ratio.
    2. Typically, the adaptation to signal statistics in ADPCM consists simply of an adaptive scale factor before quantizing the difference in the DPCM encoder.
    3. ADPCM was developed in the early 1970s at Bell Labs for voice coding
    4. feature
      1. Coding the difference between the predicted value and the sampled value by the past signal using the feature of the speech signal
      2. Reduce quantization noise by applying quantizers and predictors
      3. Maintains sound quality equal to or better than PCM and can reduce transmission speed to 32K
      4. Configuration is more complicated than PCM
  4. DM ( delta modulation or Δ-modulation )

    1. This is an A/D and D/A signal conversion technique used for transmission of voice information where quality is not of primary importance. This is the simplest form of DPCM where the difference between successive samples are encoded into n-bit data streams. In delta modulation, the transmitted data are reduced to a 1-bit data stream.
    2. Its main features are
      1. The analog signal is approximated with a series of segments.
      2. Each segment of the approximated signal is compared of successive bits is determined by this comparison.
      3. Only the change of information is sent, that is, only an increase or decrease of the signal amplitude from the previous sample is sent whereas a no-change condition causes the modulated signal to remain at the same 0 or 1 state of the previous sample.
  5. ADM ( Adaptive delta modulation )

    1. This was first published by Dr. John E. Abate (AT&T Bell Laboratories Fellow) in his doctoral thesis at NJ Institute Of Technology in 1968.
    2. This is a modification of DM in which the step size is not fixed. Rather, when several consecutive bits have the same direction value, the encoder and decoder assume that slope overload is occurring, and the step size becomes progressively larger.
    3. Otherwise, the step size becomes gradually smaller over time. ADM reduces slope error, at the expense of increasing quantizing error.This error can be reduced by using a low-pass filter. ADM provides robust performance in the presence of bit errors meaning error detection and correction are not typically used in an ADM radio design, this allows fortive-delta-modulation.
    4. ADM was later selected as the standard for all NASA communications between mission control and space-craft.
  6. ADPCM vs ADM

    division ADPCM ADM
    sampling frequency 8kHz Nyquist (6.8k) 2 ~ 4 times
    quantization system 2 ^ 4 = 16 2 ^ 1 = 2
    PCM word 4 bit 1 bit
    Transmission speed 32kbps 16kbps, 32kbps
    System configuration Complex Simple
    Channel error characteristic Word-unit encoding Bit unit encoding
  7. Intercomparison

    division PCM DPCM ADPCM DM ADM
    Sampling frequency 8kHz 8kHz 8kHz 16kHz 16kHz
    Number of Bps 8bit 4bit 4bit 1bit 1bit
    Transmission speed 64kbps 32kbps 32kbps 16kbps 16kbps
    Quantization staircase 256(28) 16(24) 16(24) 2(21) 2(21)
    System configuration usually Complex Very complex Very simple simple
    Noise Quantization Quantization Flat / Overloaded

2. Various kinds of audio format

  1. Audio CD

    • The format is a two-channel 16-bit PCM encoding at a 44.1 kHz sampling rate per channel. Four-channel sound was to be an allowable option within the Red Book format, but has never been implemented.
    • Monaural audio has no existing standard on a Red Book CD; thus, mono source material is usually presented as two identical channels in a standard Red Book stereo track (i.e., mirrored mono); an MP3 CD, however, can have audio file formats with mono sound.
  2. Super Audio CD

    • Super Audio CD (SACD) is a high-resolution read-only optical audio disc format that was designed to provide higher fidelity digital audio reproduction than the Red Book. Introduced in 1999, it was developed by Sony and Philips, the same companies that created the Red Book. SACD was in a format war with DVD-Audio, but neither has replaced audio CDs.
    • The SACD standard is referred to the Scarlet Book standard. Titles in the SACD format can be issued as hybrid discs; these discs contain the SACD audio stream as well as a standard audio CD layer which is playable in standard CD players, thus making them backward compatible.
  3. DVD-Audio

    1. DVD-Audio (commonly abbreviated as DVD-A) is a digital format for delivering high-fidelity audio content on a DVD.
    2. DVD-Audio uses most of the storage on the disc for high-quality audio and is not intended to be a video delivery format.
    3. DVD-Audio has much higher audio quality than video DVDs containing concert films or music videos. The first discs entered the marketplace in 2000.
    4. DVD-Audio was in a format war with Super Audio CD (SACD), and along with consumers' tastes tending towards downloadable music, these factors meant that neither high-quality disc achieved considerable market penetration; DVD-Audio has been described as "extinct" by 2007.
  4. XRCD ( eXtended Resolution )

    • This is a mastering and manufacture process patented by JVC for producing redbook CD. It was first introduced in 1995. An XRCD is priced about twice as high as a regular full-priced CD. JVC attributes this to the higher cost of quality mastering and manufacturing.
    • The XRCD definition refers to the mastering and manufacture process; the resulting CD and the contained data conform to the redbook standard and are encoded at 16 bits, 44.1 KHz.
    • Hence, XRCDs are playable on any compact disc player. JVC uses advanced dither algorithms (though without noise shaping) in their proprietary K2 technology to transfer the analog or digital source to physical disc.
    • The company claims to have studied how inferior CD-remastering techniques degrade the master tape sound and strives to minimize this loss. If analog, the source material is first converted to digital via JVC's patented K2 20-bit or 24-bit analog-to-digital converter.
  5. HDCD

    • High Definition Compatible Digital, or HDCD is a Microsoft proprietary audio encode-decode process that claims to provide increased dynamic range over that of standard Redbook audio CDs, while retaining backward compatibility with existing Compact disc players. Originally developed by Pacific Microsonics, the first HDCD-enabled CD was released in 1995.
    • In 2000, the technology was purchased by Microsoft, and the following year, there were over 5,000 HDCD titles available. Microsoft's HDCD official website was discontinued in 2005; by 2008, the number of available titles had declined to around 4,000. A number of CD and DVD players include HDCD decoding, and versions 9 and above of the Microsoft's Windows Media Player software on personal computers are capable of decoding HDCD.
    • HDCD encodes the equivalent of 20 bits worth of data in a 16-bit digital audio signal by using custom dithering, audio filters, and some reversible amplitude and gain encoding; Peak Extend, which is a reversible soft limiter and Low Level Range Extend, which is a reversible gain on low-level signals.
    • There is thus a benefit at the expense of a very minor increase in noise. A number of manufacturers offer players with HDCD capability. The Oppo line of players prior to 2017 all feature HDCD decoding.