Sound quality


Sound quality

Sound quality is the quality of the audio output from various electronic devices. Sound quality can be defined as the degree of accuracy with which a device records or emits the original sound waves. Sound quality is also the physical pleasure or fatigue experienced by a listener.

In a live setting the skill of the musicians, the tonal quality of their instruments, and the physical traits of the venue determine sound quality.

In a playback setting, sound quality is characterized by the same traits as in a live setting but is also affected by the recording techniques and equipment used, from the microphones at the session, through the pressing at the record or compact disc factory, to the quality of electronics and speakers used to recreate the sound in a listener's home.

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Recording

For digital recording/digital playback, sound quality depends on the range of sound which is sampled, the rate at which it is sampled, and the various conversions that occur in any sound reproduction system. With lossy codecs such as MP3 and Ogg Vorbis, sound quality is a quantifiable factor that determines how much sound data the encoder is allowed to discard in order to reduce file size. MP3-encoded sound is generally CBR, so its quality is defined by its bitrate, in kilobits per second (kbit/s). Quality of Ogg Vorbis-encoded files, which are most commonly VBR, is a decimal value ranging from –1 to 10, with –1 being suitable only for low-quality voice.

The frequency range of sound (in hertz) which the equipment is capable of sampling affects sound quality. Humans can hear frequencies ranging from about 20 Hz to approximately 20 kHz, so sampling that doesn't extend this far will have a detrimental effect on the resultant quality.

The sound wave being recorded is an analogue wave, that is, it is continuous and has some real value at every instant. The digital quantization of the analogue sound wave means that much of the continuous sound wave is not recorded. The rate at which the sound is sampled refers to the amount of information the detection equipment records for each second of the sound. The higher the sampling frequency is, the more accurate the resultant samples will be.

The conversion of sample range and sample rate between different pieces of equipment in a sound recording and reproduction system usually lower the quality of the sound.

Bit Rate

Recorded sound is stored in many formats. The need to save space on the storage device is opposed to sound quality. The smaller the sound file for a given recording the poorer the sound quality. This is not true for lossless compression methods, where the quality is preserved also in smaller sound file sizes.

The sound quality is determined by the bit rate, which is the number of samples per second times the number of bits per sample. It also depends on number of audio channels (mono, stereo, multichannel) and compression method.

The most common are lossy compression formats as MP3, OGG Vorbis, AAC and many others. For example, MP3 files commonly have a bit rate of 128 kbit/s, because it typically offers adequate audio quality in a relatively small space. MP3 has a maximum bit rate of 320 kbit/s.

By contrast, uncompressed audio as stored on an Audio CD has a bit rate of 1,411.2 kbit/s, or four times the best MP3 can do, and twelve times that of common MP3 files.

The WAV, AIFF or AU audio file formats can store audio using various bit rates, in accordance with the number of channels and compression. They often store uncompressed PCM audio with a bit rate of 1,411.2 kbit/s.

There are also lossless compression formats, such as FLAC, ALAC, Monkey's Audio and many others, which preserve quality of original uncompressed audio, but can reduce the needed storage space. Advantageously, these lossless formats can also store metadata (eg. images, title, artist, album, label, etc.) like lossy formats can, which cannot be done with pure uncompressed formats like WAV, AIFF, AU.

Delivered Audio Quality

Delivered Audio Quality, abbreviated as DAQ, is a measure of audio quality over a transmission medium. This metric is often used to quantify the quality of audio heard over a radio system. DAQ levels are defined by the following scale.

  • DAQ 1: Unusable. Speech present but not understandable.
  • DAQ 2: Speech understandable with considerable effort. Requires frequent repetition due to noise or distortion.
  • DAQ 3: Speech understandable with slight effort. Requires occasional repetition due to noise or distortion.
  • DAQ 3.4: Speech understandable without repetition. Some noise or distortion present.
  • DAQ 4: Speech easily understandable. Little noise or distortion.
  • DAQ 5: Perfect. No distortion or noise discernible.

Sources

See also


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