Audio Record Time Calculations
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.
Sound Devices has a powerful web-based record time calculator available here.
With file-based digital audio recorders, the calculation of available recording time involves three variables, track count, data rate, and storage medium capacity.
- Track count – how many concurrent audio tracks/channels are to be recorded.
- Data rate – expressed by sampling rate and bit depth for non-compressed audio and by bit rate for compressed (MP2 and MP3) audio data. Data rate determines how big the data “container” is for the audio signal. (See the calculation for determining PCM audio below.)
- Storage medium capacity – typically expressed in GB (see Drive Notes below).
The following chart indicates audio data rates expressed in hours per track (track-hours) at a given data rate for typical audio formats used with 7-Series digital recorders. When planing on recording two tracks, divide the track hours figure by two. Likewise, for four-track recording divide track-hours by four.
Uncompressed Recording Time in Track-Hours
|Data Rate (bit depth/sample rate), one track|
|16/44.1 (5.05 MB/min)||16/48 (5.49 MB/min)||24/48 (8.24 MB/min)||24/96 (16.5 MB/min)||24/192
|Storage in GB(1000 MB= 1 GB)||1||3.30||3.03||2.02||1.01||0.51|
MP3 Compressed Recording Time in Hours
Rate (bit depth/sample rate), stereo track
|64 kb/s (0.47 MB/min)||96 kb/s (0.70 MB/min)||128 kb/s( 0.94 MB/min)||160 kb/s (1.17 MB/min)||192 kb/s (1.40 MB/min)||256 kb/s (1.86 MB/min)||320 kb/s (2.34 MB/min)|
|Storage in GB(1000 MB= 1 GB)||1||35||23||17||14||11||8||7|
The chart shows that when recording 24-bit/48 kHz audio to a 40 GB hard drive the maximum amount of recording time available is 80 track-hours. If recording stereo, this yields 40 stereo hours of record time.
The 744T and 722 records have the ability to record to two devices simultaneously – an internal hard drive (2.5-inch) and CF medium. Drives can be formatted by these recorders (as FAT32 devices), or pre-formatted FAT32 formatted CF or 2.5-inch hard drives can be inserted.
Note that most storage medium now quote capacity in GB using SI units. Read more on the confusion between “GiB = gibibyte” and “GB = gigabyte”.
4 GB File Size
The 744T and 722 recorders data volumes are formatted and write to FAT32 file structures. This structure allows the drive to directly mount in a wide variety of computer platforms, including Windows, Mac OS, and Linux. Via the FireWire connection both internal drives (internal HD and CF) appear as external FAT32 volumes. (NOTE: Windows XP has a limitation on FAT32 drive formatting -XP can format a FAT32 volume to a maximum of 32 GB, although it can read FAT32 volumes as large as 2 TB.)
FAT32 has a maximum file size limitation of 4 GB. The 744T and 722 recorders will automatically split an audio file before the 4 GB size is reached and begin writing to a new file. When joined in an editing program these files match seamlessly with no samples lost. This is similar to an Audio CD which has multiple tracks on its volume. The 744T and 722 have menu-selectable file size maximums of 650 MB, 1 GB, 2 GB, and 4 GB. The 650 MB size allows the user to break an audio program into a CD-R sized file for backup to inexpensive CD-R medium.
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.