Sound Devices 744T - Manual (Page 24)

744T User Guide and Technical Information
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v. 2.67 Features and specifications are subject to change. Visit www.sounddevices.com for the latest documentation.
Relationship Between Sampling Frequency and Audio Bandwidth
The sampling frequency is expressed in samples per second (in hertz) and defines the number of
times in a 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 sampling rate is maxi-
mum analog frequency = sampling frequency/2. Higher sampling frequencies allow for wider audio
bandwidth.
The 744T generates the following sampling rates:
• 32 kHz
• 44.1 kHz
• 47.952 kHz
• 47.952kF - file stamped at 48 kHz
• 48 kHz
• 48.048 kHz
• 48.048kF -file stamped at 48 kHz
• 88.2 kHz
• 96 kHz
• 96.096 kHz
• 96.096kF - file stamped at 96 kHz
• 176.4 kHz
• 192 kHz
See Time Code section for more information about the 48.048F, 47.952F, and 96.096F rates.
Bit Depths
The 744T records at bit depths of either 16 or 24 bit. 24 bit recording provides greater dynamic range
and addition headroom for signal peaks relative to 16 bit recordings. 24 bit recording (versus 16 bit)
is a significant benefit for field production audio tracks.
Bit Depth = Available 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 accom-
modate more dynamic range. A quick estimate of maximum dynamic range capable of being rep-
resented 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, the amount of data it represents increases expo-
nentially. The majority of field recording is done with 16-bit audio, therefore, each sample is rep-
resented 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.
The 744T has 24 bit analog-to-digital converters. To obtain 16 bit recording the 744T can be set to
dither 24 bit digital signals to 16 bit. The 744T uses a proprietary pseudo-random dither routine for
accurate bit rate reduction. Dither can be defeated in the user menu. Without dither, 24 bit audio is
truncated to 16 bit, meaning the least significant 8 bits are discarded.
Once a file is recorded its sampling rate and bit depth can not be changed in the recorder. The 744T
does not perform sample rate conversion or bit depth changes. File conversion must be done in another
environment, such as an audio workstation. Alternatively, a real-time analog transfer is often performed
instead of sample rate conversion.