Question

I am trying to extract the SMPTE timecode (wikipedia) from an audio input stream in android.

As mentioned here https://stackoverflow.com/a/2099226 first step is to scan the input stream byte sequence for 0011111111111101 to synchronize. But how to do this with the AudioRecord class?

Answer 1

That answer isn’t really correct. The audio signal you are getting is a modulated carrier wave, and extracting SMPTE bits from it is a multi-step process: The raw data you get through the mike or audio in isn’t going to correspond to SMPTE timecode. Therefore, you need to decode the audio, which is not at all simple.

The first step is to convert your audio signal from biphase mark code. I haven’t implemented a SMPTE reader myself, but you know the clock rate from the SMPTE standard, so the first thing I would do is filter carefully to get rid of background noise, since it sounds like you are taking the audio in from the mike. A gentle high-pass to remove any DC offset should do and a gentle lowpass for HF noise should also help. (You could instead use a broad bandpass)

Then, you need to find the start of each clock cycle. You could do something fancy like an autocorrelation or PLL algorithm, but I suspect that knowing the approximate clock rate from from the SMPTE standard and being able to adjust a few percent up and down is good enough — maybe better. So, just look for repeating transitions according to the spec. Doing something fancy will help if you suspect your timecode is highly warped (which might be the case if you have a really old tape deck or you want to sync at very high/low speeds, but LTC isn’t really designed for this. That’s more VTC’s domain.).

Once you’ve identified the clock, you need to determine, for each clock tick, if a transition in the signal occurred at the start of the clock cycle. Each clock tick will have a transition in the middle, but a transition at the start indicates a 0 bit. That’s how BMC transmits both clock and data in a single stream. This allows you to create a new stream of your actual SMPTE data.

Now you’ve decoded the BMC into a SMPTE stream. The next step is to look for the sync code. Looking at the spec on Wikipedia and from what I remember of SMPTE, I would assert that it is not enough to find a single sync code, which may happen by accident or coincidence elsewhere in the 80-bit block. Instead, you must find several in a row at the right interval. Then you can read your data into 80-bit SMPTE blocks, and, as you read, you must continue to verify the sync codes. If you don’t see one where you expected it, start the search from scratch.

Finally, once you’ve decoded, you’ll have to come up with some way to “flywheel” because you will almost certainly not read all data correctly all the time (no checksums!). That is the nature of the beast.