[time-nuts] Timestamps in audio files?

[Tim Shoppa]

Analogies to broadcast timing are interesting, but that seems like a
non-starter today due to multiple cascaded codec latencies.

The video formats I know that use timestamps for historical reconstruction,
just recorded a human-readable timestamp onto the video itself at original
recording time.

Jim, I agree that playing games with the timestamp cause problems with many
players that expect the start to be "00:00:00".

Certainly embedding time info into the transport layer is a possibility.
I'm more interested in the "file" layer today.

I remember to the last time I had to extensively work with analog
recordings with IRIG-type timestamps. The playback console had a LED
playback-clock readout and it was very impressive because with
variable-speed analog playback the LED readout's IRIG decoder would
maintain lock to the timecode. (It must've been one of the
amplitude-modulated IRIG codes now that I think about it, because it would
show hundredths of a second - IRIG-B? - and sometimes we would record it on
a polaroid with a scope and count off fractions of 0.01s. Or maybe it was
0.001s.)

Tim N3QE


On Mon, Dec 21, 2015 at 12:40 PM, Bob Camp <kb8tq at n1k.org> wrote:

> Hi
>
> I sort of suspect that if there *was* a system “broadcasting” time over
> the internet
> (other than NTP) we all would be fooling around locking up oscillators to
> it …
>
> Yes, streaming and time stamping are not the same thing. These days
> though, the
> two probably get crossed between a lot.
>
> Bob
>
> > On Dec 21, 2015, at 10:22 AM, Jim Lux <jimlux at earthlink.net> wrote:
> >
> > On 12/21/15 3:19 AM, Tim Shoppa wrote:
> >> As an adjunct to the thread about timestamped samples of LORAN
> >> transmissions...
> >>
> >> Are there any standard consumer-type audio file formats, that support
> >> absolute time time/datestamps? Would not have to be done continuously,
> but
> >> something like a time and date stamp inserted nearest each sample on a
> >> second boundary.
> >>
> >> I have worked with some analog tape audio formats in the past where
> >> IRIG-type timestamps were written on a separate channel or on a
> subcarrier.
> >>
> >> I know of many proprietary digital recording applications that make
> WAV's
> >> or MP3's or proprietary codec formats, where the filename includes a
> >> timestamp. Much more interested in standard formats where the timestamp
> is
> >> embedded in the file itself.
> >>
> >
> > For RF recordings, VITA49 has a standard for timestamps in the packet
> headers (4 flavors of epoch, multiple flavors of time format and precision)
> >
> > Video file formats seem to draw from older time code things like SMPTE
> and are "relative" (so you're always fooling around trying to figure out
> the offsets).  I spent a few days earlier this year trying to put absolute
> time subtitles on video files using all manner of tools, and it was
> frustrating (ffmpeg, vlc, etc.. all were to no avail).  Trying to put UTC
> time into embedded timecode was also pretty unproductive (most tools don't
> like to see the first frame occurring at a time very different from
> 00:00:00:00)
> >
> >
> > In fact, in the music file world (e.g. MIDI) you see references to
> absolute and relative time, and there, they are really talking about time
> measured in seconds vs time measured in beats; e.g. whether the duration of
> something  is 1 second, or 2 quarter notes, which might be the same if the
> tempo is 120bpm.
> >
> >
> > You might look for solutions for people trying to synchronize multiple
> multimedia streams delivered over the internet (e.g. slides and
> accompanying narration or music) because they actually have a need for
> "show this slide at time HH:MM:SS and play this sound at HH:MM:SS" kind of
> synchronization.
> >
> > I suspect, though, that this kind of info gets encapsulated in the
> transport layer, rather than the underlying files holding the info.
> >
> >
> >
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