问题
So I have been trying to wrap my head around mpeg-ts timing, and the PCR (program clock reference). I understand that this is used for video/audio synchronisation at the decoder.
My basic understanding so far is that everything is driven by a 27 Mhz clock (oscillator). This clock loops at a rate of 27 Mhz, counting from 0 - 299 and keeps repeating itself. Each time this "rollover" from 299 back to 0 occurs, then a 33-bit PCR counter is incremented by 1. In effect, the 33-bit PCR counter is therefore itself running at a rate of 90 kHz. So another way of saying this is that the 27 Mhz clock is divided by 300, giving us a 90 kHz clock.
This 90 kHz clock is then used for the 33-bit PCR counter.
I understand that historically 90 kHz was chosen because mpeg-1 used a 90kHz timebase. [see source here]
Anyway... I have read that the PCR 33-bit count values range from 0x000000000 all the way through to 0x1FFFFFFFF. And according to this, it shows what these values mean in terms of time as we humans understand it (Hours, Mins, Secs, etc): 00:00:00.000 (0x000000000) to 26:30:43.717 (0x1FFFFFFFF)
So ultimately, my question is relating to how do these hex codes get translated into those time stamps. What would the equations be if someone gave me a hex code, and now I need to reproduce the time stamp?
I would appreciate any help :)
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I am closer to an answer myself. Looking at the range from 0x000000000 to 0x1FFFFFFFF, this is basically 0 to 8589934591 in decimal. Since the PCR clock is 90Khz, to get the number of seconds it takes to go from 0 to 8589934591 we can do 8589934591/90000 which gives us 95443.71768 seconds.
回答1:
Unless you are creating a strict bitrate encoder for broadcast over satellite or terrestrial radio, the PCR doesn't matter that much.
Scenario: You are broadcasting to a wireless receiver with no return channel, The receiver has a clock running at what it thinks is 90000 ticks per second. Your encoder is also running at 90000 tickets per second. How can you be sure the receiver and the broadcaster have the EXACT same definition of a second? Maybe one side is running a little fast or slow. To keep the clocks in sync, the encoder sends the current time occasionally, This value is the PCR. For example, if you are broadcasting at 15,040,000 bits per second, the receiver receives a 188 byte packet every 0.0000125 seconds. Every now and then (100 ms) the encoder will insert its current time. The receiver can compare this time to its internal clock and determine if is running faster or slower than the broadcast encoder. To keep the strict 235,000 packets per second (15,040,000/(188*8) = 235,000) the encoder will insert null packets. On the internet, the null packets take bandwidth, and have no value, so they are eliminated. Hence the PCR has almost no value anymore because its time is no longer relative the the reception rate.
To answer your question. Set the 27hz value to zero, use a recent DTS minus a small static amount (like 100ms), for the 90khz value.
来源：https://stackoverflow.com/questions/53401917/how-to-generate-timestamps-from-the-33-bit-pcr-count