One day you'll need to include real-time UTC timestamps in audio. It's useful when reconstructing events from long, unsupervised surveillance microphone recordings, or when constantly monitoring and logging radio channels.
There's no standard method for doing this with WAV or FLAC files. One method would be to log the start time in the filename and calculate the time based on audio position. However, this is not possible with voice-activated or squelched recorders. It also relies on the accuracy and stability of the ADC clock.
I'll take a look at some ways to include an accurate timestamp directly in the in-band audio.
Least significant bit
Time information can be encoded in the least significant bit (LSB) of the 16-bit PCM samples. This "steganographic" method requires a lossless file format and lossless conversions. The script below truncates all samples of a raw single-channel signed-integer PCM stream to 15 bits and inserts a 20-byte ISO 8601 timestamp in ASCII roughly every second, preceded by a "mark" start bit. When played back, the LSB can be zeroed out to get rid of the timestamps. The WAV can also be played as such; the "ticking" sound will be practically inaudible at an amplitude of −96 dB. The outgoing PCM stream is then sent to SoX for WAV encoding.
Note that the start bit of the timestamp will mark the moment the sample reached this script, and it could differ hundreds of milliseconds from the actual moment of reception at the microphone. Also, the timestamp does not mark the start of a second, but is rather timed by an arbitrary sample counter. One could also poll and write the timestamps in a continuous manner.
The above script could be modified to interface with my squelch script, by only inserting timestamps when squelch is not active. The resulting audio could then be efficiently encoded as FLAC.
lsb-time-read.pl reads back the timestamps, also printing the sample position of each. Below is a sound sample of a clean signal followed by a timestamped one.
Lossy compression, by definition, does not retain the numeric values of samples, so they can't be treated as bit fields. Instead, we can use an analog modulation scheme like binary FSK. MP3 and Ogg Vorbis encoders will, at a reasonable bit rate, retain the structure of a sufficiently slow FSK burst. This method will work even if the timestamping phase is followed by an analog conversion.
Using the ultrasonic part of the spectrum comes to mind; but unfortunately such high frequencies are mainly ignored by a LPF at the encoder. However, we can use the higher end of the remaining spectrum and filter it out afterwards, if the recording consists of narrow-band speech. In the case of squelched conversation, we could write the timestamp only in the beginning of each transmission. This way it could even be in the speech frequencies.
fsk-timestamp.pl embeds the timestamps into PCM data; they can be read back using
minimodem --rx --mark 11000 --space 13000 --file stamped.wav -q 1200.
A sound sample follows.