If analogue TV is all now digital...

[evilclive]

As far as I'm aware, terrestrial broadcasts are sent down broadband or equivalent cables from the broadcasting studios to the regional transmitters: the days of ferrying videotapes around are long since gone. I'd be very surprised if the signals aren't compressed in MPEG-2 format for this purpose, and then the transmitters either multiplex them (for digital TV signals) or decompress them (for analogue TV broadcasts).

The upshot is that there is no longer any advantage in capturing the analogue signal with a lossless codec, when the best results you can possibly get would be obtained by capturing the pre-compressed digital signal.

For the first time in over a year, I want to record two terrestrial channels simultaneously. I don't really want to buy a second Freeview dongle for a one-off, so I'm planning to capture one of the programmes on an old analogue Pinnacle box. My box pre-dates universal USB-2, so it compresses the input signal in hardware, creating a MPEG-2 stream that fits the bandwidth limitations of USB-1.1.

Last time I used it, I was capturing at 8Mbit/s, and found that the recording occupied about 4Mbit/s of hard disk space - roughly equivalent to the bandwidth of Freeview transmissions.

My question: if the MPEG-2 signal is compressed on two occasions (once by the broadcaster, then decompressed by the broadcaster for the analogue network, then compressed again at a higher bit rate by my capturing device), will this create MPEG artifacts over and above what there would be in the original MPEG-2 stream (that is presumably being broadcast on digital terrestrial TV)?

[neuron2]

My question: if the MPEG-2 signal is compressed on two occasions (once by the broadcaster, then decompressed by the broadcaster for the analogue network, then compressed again at a higher bit rate by my capturing device), will this create MPEG artifacts over and above what there would be in the original MPEG-2 stream (that is presumably being broadcast on digital terrestrial TV)? Read this:

http://www.broadcastpapers.com/whitepapers/AvidPreserveVideoQuality.pdf

[davidhorman]

I haven't read that PDF, so sorry if I'm duplicating...

AFAIK, the pictures sent to the transmitters are at a higher bitrate than what gets broadcast - typically 20mbit, I believe. It's decompressed for analogue broadcast and recompressed for digital broadcast, so in some senses the analogue signal is better (of course, it suffers from colour bleeding, dot crawl, and interference...)

Last time I used it, I was capturing at 8Mbit/s, and found that the recording occupied about 4Mbit/s of hard disk space - roughly equivalent to the bandwidth of Freeview transmissions.

If you're suggesting that it could only fill 4Mbit/s of data, because that's what the original stream was, I'd be very surprised, as it's been converted to analogue. I would have expected it to find enough detail to use up 8Mbit/s, but that's a guess really.

David

[evilclive]

If you're suggesting that it could only fill 4Mbit/s of data, because that's what the original stream was, I'd be very surprised, as it's been converted to analogue. I would have expected it to find enough detail to use up 8Mbit/s, but that's a guess really.

The TV capture box I'm using dates from 2002 or thereabouts. It predates universal ownership of computers with USB2 and 1GHz+ CPUs, so it encodes the MPEG in hardware.

I don't know how powerful the signal processor is inside such an old box. That might be limiting factor, rather than the amount of usable information in the analogue signal.

I used it to record BBC2 last night. The recording was 90.5 minutes in duration, yielding a file of size 3564MB: an average bit rate of 5.25 Mbit/s for video and audio together.

The Pinnacle box was on the DVD-quality preset: MPEG-2 VBR at up to 6.00 Mbit/s.

BBC2 is one of the high-bandwidth digital channels, broadcasting on Freeview at up to 6.5 Mbit/s video plus audio. (Most channels, apart from BBC 1, 2 and 3, are broadcast at a much lower bit rate, to squeeze more Freeview channels into the available spectrum.)

To answer this question definitively, I'll need to record something from BBC1 at about 15 Mbit/s, and measure the average bit rate.

[evilclive]

To answer this question definitively, I'll need to record something from BBC1 at about 15 Mbit/s, and measure the average bit rate.

I've just done this. The programme I chose was originally screened in 1981, so the BBC's master tape will certainly be analogue.

The MPEG-2 file is 1724MB in size, with a play length of 28:58 minutes and an average bit rate of 7.93 MBit/s.

This is much less than the nominal recording setting I used (15 Mbit/s with USB 2), but is consistent with davidhorman's estimate above.

[davidhorman]

It wasn't an estimate - I actually meant it would probably find enough detail to fill up to your selected bitrate (which at that time was 8mbit). So that was just a coincidence!

David