ADPCM increases and decreases the magnitude that the difference values represent as the sound signal amplitude changes.Īs with graphic/images files, different systems use different file formats for audio. The Compact Disc-Interactive format uses ADPCM (Adaptive DPCM), to achieve better compression.
Since this difference tends to be small, less bits are required to store the difference values. DPCM only stores a measurement of the difference between the last sample and the next. Because sampled sound will differ little from one fraction of a second sample to the next the DPCM (Differential PCM) technique is widely used. The CD Audio standard (PCM) is commonly termed linear PCM since it performs no compression, storing each value as a separate 16-bit value. PCM is the standard method employed in the CD Audio format.įor various reasons audio data does not easily yield high compression rates when standard textual compression methods are employed.
SAMPLE QUALITY SOUND REFERENCE CODE
This two step process just described for sampling and quantizing sound digitially is termed Pulse Code Modulation (PCM). Decreasing the quantization to 8-bits, to save 50% of the required storage, decreases the S/N ratio to about 50 dB, approximately the same quality as AM radio. S/N ratios must be greater than 70 dB to prevent backgound noise from becoming audible. CD Audio has a theoretical S/N ratio of about 96 decibels (dB), with actual systems achieving S/N ratios in the low 90 dB. The higher the S/N ratio the better the sound. The S/N ratio is a ratio between the difference of the highest and lowest frequencies to the average superimposed noise (white noise or static). In audio theory this is referred to as the signal-to-noise ratio ( S/N). This process involves dealing with the error between the sampled discrete values and the actual continuous sound, termed quantization error. In simple terms, quantization can be viewed as converting real (continuous sound), values into integer (discrete sound) values. CD audio, the most common quantization strategy uses 2 bytes (16 bits) capable of representing 65,536 discrete levels. The number of distinct sound levels that can be represented is determined by the number of bytes used to store the quantization value. The process of converting a sampled sound into a digital value is termed quantization. To reproduce high fidelity music at CD audio quality the sampling rate must be 44.1 kHz, giving a playback rate of 22 kHz, which is just above the limit of human hearing. Most natural world sounds and medium fidelity music can be reproduced at 11kHz with acceptable losses of fidelity (approx. This requires a sampling rate of only 11kHz. Human speech can be effectively reproduced at a rate of 5.5 kHz (kilo-Hertz).
While in practical applications this maximum loss of fidelity would not likely occur, unacceptable errors due to the Nyquist effect would be exhibited.Īlthough many sampling rates exist, only the most popular and most common will be discussed. In a worst-case scenario with only one sample per period, instead of two samples as the above graph depicts, the reproduced sound might be played back as a continuous tone. The reason for this discrepancy is due to the Nyquist Effect (or Nyquist Theorem). In fact the sampling rate must be two times the playback rate. In deciding upon a sampling rate one must be aware of the difference between playback rate and capturing (sampling), rate. Knowing the type of audio to be sampled and the intended purpose of the audio allows for a reasonable choice of sampling rate. This requires a tradeoff to be made between the quality of the sound and the storage requirements. For example, the CD audio format can hold 600 million characters of text, but only 74 minutes of uncompressed music. Digital sound sampled at high fidelity rates requires massive storage. The higher the sampling rate the higher the fidelity and the higher the storage requirements. There exists a direct relationship between the sampling rate, sound quality (fidelity), and storage space. Sampling involves the rate at which the converted sound is captured. Digitization of analog sounds is composed of two phases: 1. The process of converting natural analog sound into discrete digital sound is digitization. Natural sound is the result of a stream of continuous changes of vibrations in the atmosphere.
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