Audio Systems: audio systems question, analog to digital conversion, analogue signal

Question
hello, thanks in general for doing that. i have few questions unanswered and i would like to help me,

1) why digital audio data compression in general is necessary?

2) why basic methods of data reduction not generally adequate for digital audio?
3)what happens before an analogue signal is converted into binary code(before entering the ADC), why is it needed? are there any serious consequences? thanks again

Answer
Digital compression isn't necessary. If you burn in wav it will be uncompressed but still flawed. 16 bit 44.1 is a compromise to begin with. There is a portion of the wav file that gets truncated and there are lost bits (called least significant bits). That is why SACD was created. In SACD the bit rate is extended to 20bits and sometimes 24. Then there is 24/192 which is typical of DVD audio. It is as high an  audio quality as can be achieved today. There are audiophiles who prefer high quality vinyl reproduction at an astronomical cost. That is what 24/192 has accomplished. As far as digitally compressing, only few of us, myself included, can hear the difference between a compressed file at a high sampling & bit rate.

Compression is used to get more audio data onto a hard drive and to send via email. Lossless is the equivalent of wav and I described in a previous anwer the advantage of using lossless vs wav. Lossless will always be better than wav (take my word for it). Any professional will concur unless the rate is higher that 16/44.1.

As far as your questions about analog to digital conversion there are many books available on the topic. Look for explainations of dithering and artifacts.

To get you started:


     Sample rate: The rate at which the samples are captured or played back, measured in Hertz (Hz), or samples per second. An audio CD has a sample rate of 44,100 Hz, often written as 44 KHz for short. This is also the default sample rate that Audacity uses, because audio CDs are so prevalent.
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     Sample format or sample size: Essentially this is the number of digits in the digital representation of each sample. Think of the sample rate as the horizontal precision of the digital waveform, and the sample format as the vertical precision. An audio CD has a precision of 16 bits, which corresponds to about 5 decimal digits.

Higher sampling rates allow a digital recording to accurately record higher frequencies of sound. The sampling rate should be at least twice the highest frequency you want to represent. Humans can't hear frequencies above about 20,000 Hz, so 44,100 Hz was chosen as the rate for audio CDs to just include all human frequencies. Sample rates of 96 and 192 KHz are starting to become more common, particularly in DVD-Audio, but many people honestly can't hear the difference.

Higher sample sizes allow for more dynamic range - louder louds and softer softs. If you are familiar with the decibel (dB) scale, the dynamic range on an audio CD is theoretically about 90 dB, but realistically signals that are -24 dB or more in volume are greatly reduced in quality. Audacity supports two additional sample sizes: 24-bit, which is commonly used in digital recording, and 32-bit float, which has almost infinite dynamic range, and only takes up twice as much storage as 16-bit samples.

Playback of digital audio uses a Digital-to-Analog Converter (DAC). This takes the sample and sets a certain voltage on the analog outputs to recreate the signal, that the Analog-to-Digital Converter originally took to create the sample. The DAC does this as faithfully as possible and the first CD players did only that, which didn't sound good at all. Nowadays DACs use Oversampling to smooth out the audio signal. The quality of the filters in the DAC also contribute to the quality of the recreated analog audio signal. The filter is part of a multitude of stages that make up a DAC.

How does audio get digitized on your computer?

Your computer has a soundcard - it could be a separate card, like a SoundBlaster, or it could be built-in to your computer. Either way, your soundcard comes with an Analog-to-Digital Converter (ADC) for recording, and a Digital-to-Analog Converter (DAC) for playing audio. Your operating system (Windows, Mac OS X, Linux, etc.) talks to the sound card to actually handle the recording and playback, and Audacity talks to your operating system so that you can capture sounds to a file, edit them, and mix multiple tracks while playing.

Standard file formats for PCM audio