Before file-based recording technology was introduced for portable recording, planning storage needs was straight forward—buy enough medium (tape) to cover the recording time. In analog, a 7-inch reel at 7-1/2 ips lasts 33 minutes. A 120 minute DAT tape lasts…120 minutes, whether recorded in mono or stereo.
With file-based digital audio recordering, the calculation of available recording time involves three variables:
- Track count – how many concurrent audio tracks/channels are to be recorded.
- Data rate – the amount of data generated per second. Compressed formats express the data rate directly (Usually in kilobits per second). Data rate for uncompressed formats is a function of sample rate, bit depth, and track count.
- Storage capacity – typically expressed in GB.
The following calculator will find the amount of storage space required given a time value, or the amount of time available with a given amount of storage space.
Note that most storage medium now quote capacity in GB using SI units. Read more on the confusion between “GiB = gibibyte” and “GB = gigabyte”.
Uncompressed digital audio is expressed numerically by two measurements, bit depth and sampling frequency, such as 16-bit/48 kHz. These two numbers are used to compute the data rate of uncompressed audio.
Bit Depth = Dynamic Range.
Bit depth defines the digital “word length”used to represent a given sample. Bit depth correlates to the maximum dynamic range that can be represented by the digital signal. Larger bit depths theoretically yield more dynamic range. A quick estimate of maximum dynamic range capable of being represented by a given word length is dynamic range ~= no. of bits x 6 dB. Bit depth is an exponential measure (exponent of 2), so as bit depth increases, theamount of data it represents increases exponentially. The majority of field recording is done with 16-bit audio, therefore,each sample is represented by a digital word of 2^16 (65,536) possible values. 24-bit audio has a word length of 2^24 (16.7 million) possible values per sample.
Sampling Frequency = Audio Bandwidth.
The sampling frequency is expressed in samples per second (in Hz) and defines the number oftimes in second that the analog audio signal has been measured Sampling frequency determines the audio bandwidth, or frequency response, that can be represented by the digital signal. A quick estimate of the maximum bandwidth capable of being represented at a given sample rate is max analog frequency= sampling frequency / 2. Higher sampling frequencies theoretically yield wider audio bandwidth. For example, the DV25 format can sample at 48, 44.1, or 32 kHz. At its highest sample rate each second of audio is made up of 48,000 samples.
Audio Data Rate = Bit Depth x Sampling Frequency.
In the example below the data rate of a single 16-bit/48 kHz audio stream is computed in mebibytes per minute. Division by 1048576 converts from bits to mebibits. Division by 8 converts from mebibits to mebibytes; multiply by 60 converts seconds to minutes.
(((16 x 48000) / 1,048,576) / 8) x 60 = 5.49 MiB/min
When digital audio is compressed using some form of lossy, perceptual process such as MPEG2-Layer3 (MP3 audio), Windows Media encoding (WMA), ATRAC encoding (used in MiniDisc), AAC (MPEG-4 audio), or others – it can have a significant reduction in its data rate. Compressed audio has enabled the practical distribution of audio over the low speed data pathways.