Originally Posted by waynomo
No problem. Your TiVo HD is reading from a specific point on the disc. You press pause and then switch to the other tuner. You are not really watching from the tuner. The program is being written to disc first. So now you are reading from another area of the disc. When you press the "Live TV" button to change back to the other tuner, you are really just accessing another part of the disc. It doesn't remember the exact spot where you were, but a segment. Doing this instead of having to index every frame of video makes switching quicker.
That's not quite right. The problem here is caused by the temporal compression used by MPEG-2. MPEG-2 video can only start decoding on an I frame. The frequency of I frames in the original video stream is determined by the GOP size set during encoding. Can be anywhere from one every 1/2 second to one every 2-3 seconds. When you pause a recording it shows the exact frame you were on, even if it's a P or B frame, because it has decoded the video to that point and is capable of displaying that frame. However when you switch tuners you flush the decode buffer which means the frame you were displaying can no longer be decoded. So when you switch back to that tuner it has to jump to the nearest I frame to start decoding again. There are a few things that can mess up that jump. First if the GOPs are really long then the nearest I frame might be 2-3 seconds away from where you were paused. Second there is no map telling the software where exactly the nearest I frame is, so they have to guess. If it's anything like VideoReDo they guess by using the average bitrate of the entire stream. However if the bitrate fluctuates a lot over the course of the stream it can throw off that guess.
In any case you were right that it's no problem.
OP - The reason you're likely just starting to see this is because a lot of cable companies and networks have started adjusting their encoding parameters to save bandwidth. These changes usually include using longer GOPs and more variable bitrates, both of which can effect the pause point as mentioned above. Unlike the old S2 TiVos the HD units record the MPEG-2 stream as-is, so they don't have any control over the encoding parameters and have to deal with what they get as best they can.