Understanding Audio Quality: Bit Rate, Sample Rate
Audio Quality is the accuracy and enjoyability of the audio which the user can listen from an electronic device. Audio quality depends upon the bit rate, sample rate, file format and encoded method. It also depends on the ability of the encoder to get the important bits right.
Bit Rate refers to the audio quality of the stream. It is measured in Kilobitspersec(kbps or k). Bit rate is no of bits (data) encoded per second or the no. of bits transmitted or received per second. Higher the bit rate with more sampling rate, requires high bandwidth and produces good audio quality. Low bit rates refer to smaller file size and less bandwidth with a drop in audio quality. For good quality music usually 64–128kbps(96kbps+ recommended) bit rate is preferred.
Sample Rate is the number of samples per unit time. A sample is a measurement of signal amplitude and it contains the information of the amplitude value of the signal waveform over a period of time. The sample rate is also called as sample frequency, higher the sample frequency obtains a signal which is similar to original analog signal for good audio quality. The file size depends upon the sample frequency. The bit depth refers to no. of bits in each sample, determines the maximum signal to noise ratio. The bit depth may be 16-bit, 24-bit, 32-bit, for audio CD 16-bit is preferred. The sample rate is measured in hertz(Hz). According to Nyquist Sampling theorem the sampling frequency to produce the exact original waveform should be double the original frequency of the signal. The human hearing bandwidth is 20Hz-20kHz, the audio sampled can be at the rate above 40kHz.(Usually 44.1KHz is preferred).
Bandwidth is the speed that you can send data or receive the data.It depends upon the bit rate at which the data is send or received.For more bit rate the bandwidth consumed is more for which the cost to broadcaster will increase. As the bit rate increases the amount of data streamed per second increases at a good sampling rate to produce the replica of analog signal with more bit depth(16 for audio) thus increasing the bandwidth and file size to produce the best audio quality. Some of the bit rate and sample rate preferred are given below: For MP3format the Mp3 streaming bit rates and sample rate for stereo may range from 96–320 kbps/44.1–48KHz, the preferred bit rates are 128Kbps/44.1KHz, 96Kbps/44.1KHz. The audio quality depends on the encoded format, it is difficult to determine which encoded format at chosen bit rate sounds good. For example the bit rate at 128kbps Mp3 format sounds the same quality as AAC format at 96kbps/44.1KHz(apple lossy compressed format for itunes).
Bit Rate calculation(Uncompressed Format) bit rate=bitspersample(16-bit or 24-bit)*samplespersec(44.1KHz-48KHz)*no.of channels. Example 16-bit 48Khz strereo contains bitrate of 1.5Mb/sec. File size calculation For Uncompressed Format file size=((bitspersample(16-bit or 24-bit)*samplespersec(44.1KHz-48KHz)*no.of channels*duration(no.of sec the music played))/8. Example 16-bit 44.1Khz stereo for 60 min the file size is 630MB. For Compressed Format File size=((bit rate in kbps)*(length of the audio in sec))/8 Example 16 bit 44.1Khz stereo for 60min at 128Kbps the file size is 10.8MB For live streaming the bandwidth required can be calculated using the formula: bandwidth=listeners*bit rate*length(audio length per day)*no. of days. The bandwidth required for bit rate of 128Kbps is 57.6MB/hr. Table Representing the bandwidth for various Bitrates The length of the file is taken as 60min(180 sec)