On 12/2/16 9:20 AM, Chris Caudle wrote:
On Thu, December 1, 2016 8:19 am, Tim Shoppa wrote:
Is there a common digital audio format that embeds in the digital
stream, a timestamp marker of real-world-clock-time that the
audio was recorded at?
At my "day job" we have many digital "system of record" phone and radio
recording systems. The best they do, is to timestamp the filenames they
generate with the start time.
You mention timestamping files, and also digital stream. Are you looking
for a transport protocol, or a file format?
For a file format, Broadcast WAV described in EBU tech report 3285 has a
field for origination time, with a resolution of 1 second, and a time
reference which as I understand is the location of the first sample
referred to the previous midnight given in sample position as a 64 bit
number.
Presumably this give some ambiguity of the location of the ending samples
based on the accuracy of the sample clock originally used to capture the
samples.
If you need transport time stamps, then the audio-over-IP protocols use
PTP as the reference clock, so you get explicit description of the audio
sample location referenced to the PTP epoch.
If you want to get away from existing "audio" formats (realistically,
SMPTE/AES/EBU would be a better choice in your business, though), you
can also look at VITA-49 Virtual Radio Transport (VRT)- it's a format
for digitized samples from a radio with explicit time tags in the
messages - the field is 2 32 bit words, seconds and picoseconds.
Although I know the latter would not be sufficient for some time-nuts,
but there is a provision for user defined extension fields in periodic
context packets where you could provide clock offsets and calibration
factors.
If anyone needs it, I've got python and Matlab/Octave code to read and
decode VRT format files, although I don't support ALL possible sample
encodings. The standard contemplates all sorts of strange packing and
formats - I do signed 16 bit and signed 24 bit.
In any case, it's a lightweight protocol - the header is as short as 4
32 bit words (header word, stream id, time coarse, time fine) followed
by however long your data packet is. And you can leave out stream id or
time if you like.
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