Capturing from DVR

[dkelly701]

I have Time Warner Digital Cable and Time Warner’s DVR. When the coaxle comes out of the box, I have it split and 1 cable goes to my TV and the other goes to my capture device. Will the quality of my capture suffer if I record a program to the DVR and then play it back for capture, as opposed to capturing it "live?"

I have assumed that the digital signal that my box receives is recorded on the DVR as a data file identical to the original broadcast and when replayed, it would be identical to the original signal. In other words, if it is not converted to analog, then such variables as bitrate don't come into play.

Am I thinking about this incorrectly?

[unclebud]

What are your connections from the Cable box to your DVR and then to your capture device?
S-Video, Component, Composite, HDMI/DVI or coax cable?

[edDV]

I have Time Warner Digital Cable and Time Warner’s DVR. When the coaxle comes out of the box, I have it split and 1 cable goes to my TV and the other goes to my capture device. Will the quality of my capture suffer if I record a program to the DVR and then play it back for capture, as opposed to capturing it "live?"

I have assumed that the digital signal that my box receives is recorded on the DVR as a data file identical to the original broadcast and when replayed, it would be identical to the original signal. In other words, if it is not converted to analog, then such variables as bitrate don't come into play.

Am I thinking about this incorrectly?

Coax out of the DVR is lowest quality RF (ch 3 or 4 NTSC). Better is composite (RCA), better still is S-Video. If you have any other outputs let us know. Some HD cable boxes have MPeg2 TS streams over IEEE-1394.

I have assumed that the digital signal that my box receives is recorded on the DVR as a data file identical to the original broadcast and when replayed, it would be identical to the original signal. In other words, if it is not converted to analog, then such variables as bitrate don't come into play.

Most DVRs offer different compression rates to disk. Best setting will be close to the 525x480 2-8Mbps MPeg2 that Time Warner uses for "digital" channels. Everything usually goes to analog for output.

If your capture card is digital QAM capable (and time-warner allows it) you can split the RF upstream of the DVR and tune digital channels directly at your digital capture card. It is unlikely the DVR outputs anything but analog.

[dkelly701]

The cable box and DVR are one integrated unit. All incoming channels are digital. Thus I assume that the digital signal is never converted to analog before being recorded to the hard drive. The conversion takes place when it is outputted to the TV/Capture device.

I know the coaxle cable is the least desirable way to go, but the distance from the cable box to my computer makes this the best option.

[edDV]

The cable box and DVR are one integrated unit. All incoming channels are digital. Thus I assume that the digital signal is never converted to analog before being recorded to the hard drive. The conversion takes place when it is outputted to the TV/Capture device.

...

Correct assumption*. DVR menus offer a selection of bitrates (quality vs capacity) for disk recording.


* although most cable systems are a mix of analog, Mpeg2 (cable SD and HD) and local channel DTV MPeg2.

[Seeker47]

Coax out of the DVR is lowest quality RF (ch 3 or 4 NTSC). Better is composite (RCA), better still is S-Video. If you have any other outputs let us know. Some HD cable boxes have MPeg2 TS streams over IEEE-1394.

How is it possible to avoid having plain ol' coax cable somewhere in the chain -- especially upstream of the box, where your signal will be compromised at the earliest stage ? I mean, that's how it comes in from the pole, outside -- right ? I haven't seen any cable installers -- at least not any from the cable Co. -- using some super deluxe cable, where it comes into your house or apartement. When HD service becomes much more common, you'd think they should be moving to something better than that standard, cheap, narrow guage white cable . . . .

[dkelly701]

[quote="edDV"]

The cable box and DVR are one integrated unit. All incoming channels are digital. Thus I assume that the digital signal is never converted to analog before being recorded to the hard drive. The conversion takes place when it is outputted to the TV/Capture device.

...

Correct assumption*. DVR menus offer a selection of bitrates (quality vs capacity) for disk recording.

I guess I still don't quite understand. Time Warner Cable advertises that all channels are now digital in my area. I believe that this is correct because I can tell a difference in the picture quality from before.

If the incomming signal is digital, it is essentially a data steam. If the hard drive on the cable box takes that data stream and records it, how can bitrate come into play? I thought bitrate was only applicable to an analog capture. I would think that it was more like a firewire transfer of a data file from a digital camcorder to you computer. In that instance you get a perfect copy of the recording on your camcorder.

By the way, I can find no settings on my Time Warner cable box/DVR that allow me to change the quality of the recordings.

[SHS]

Time Warner Cable DVR box don't have any option for PQ setting your stuck with it setting which is 6Mbps recording all the time I know I have one newer SA 8300.
But my main setup SageTV Server with WinTV-PVR 500 and USB-UIRT for channel and 3 250GB harddrives.
I don't know about where you live at dkelly701 but my TWC suck big with Dot Crawl no matter what input it is not count the few other odd ball video artifact I see and I'm so happy that I'm moving and looking forword go back Dishnetwork that plan but we see.

[unclebud]

The digital video has already been rendered at a specific bitrate before it leaves Time Warner to come to your cable box.
Digital channels are then scrambled until your cable box descrambles the signal and gives you analog out (Component, S-Video and Composite) and digital out (DVI/HDMI) to your TV.
As for running coax from your digital source DVR to a capture device, if you run a long S-Video cable with Stereo RCA audio cables will give you a much clearer video picture and audio then to go through the modulation and then demodulation in a second tuner (video capture device).

[edDV]

Coax out of the DVR is lowest quality RF (ch 3 or 4 NTSC). Better is composite (RCA), better still is S-Video. If you have any other outputs let us know. Some HD cable boxes have MPeg2 TS streams over IEEE-1394.

How is it possible to avoid having plain ol' coax cable somewhere in the chain -- especially upstream of the box, where your signal will be compromised at the earliest stage ? I mean, that's how it comes in from the pole, outside -- right ? I haven't seen any cable installers -- at least not any from the cable Co. -- using some super deluxe cable, where it comes into your house or apartement. When HD service becomes much more common, you'd think they should be moving to something better than that standard, cheap, narrow guage white cable . . . .

Upstream from the cable box (the coax from the pole) is a mix of 6 MHz analog channels (usually the two digit channels) and MPeg2 digital channels modulated into more 6MHz analog channels using QAM modulation. Cable internet is modulated into a pair of 6MHz analog channels. Digital telco is also modulated into even more 6MHz analog channels.

Newer wider bandwidth cable systems include both analog and MPeg2 versions of the lower channels (hence the "all digital" hype). In reality, all digital information is carried over analog RF for cable or DBS.

Coax cable is fine for this purpose. The coax that comes out of a cable box is a low quality legacy single analog RF channel (3 or 4) just like a VCR.

[edDV]

[quote="dkelly701"]

The cable box and DVR are one integrated unit. All incoming channels are digital. Thus I assume that the digital signal is never converted to analog before being recorded to the hard drive. The conversion takes place when it is outputted to the TV/Capture device.

...

Correct assumption*. DVR menus offer a selection of bitrates (quality vs capacity) for disk recording.

I guess I still don't quite understand. Time Warner Cable advertises that all channels are now digital in my area. I believe that this is correct because I can tell a difference in the picture quality from before.

If the incomming signal is digital, it is essentially a data steam. If the hard drive on the cable box takes that data stream and records it, how can bitrate come into play? I thought bitrate was only applicable to an analog capture. I would think that it was more like a firewire transfer of a data file from a digital camcorder to you computer. In that instance you get a perfect copy of the recording on your camcorder.

By the way, I can find no settings on my Time Warner cable box/DVR that allow me to change the quality of the recordings.

Comcast has different bit rates on the cable for different MPeg2 SD channels that range from 2 to >6Mb/s. These vary by type of channel where low motion channels like CNBC will get lower bitrate and premium channels like HBO get 6 or more Mb/s. The bitrate used on the DVR HDD may or may not be the same as that delivered by the cable.

Cable systems differ locally for both Comcast and TimeWarner. Older low bandwidth systems must compress more to cram more "digital" channels into the available 6MHz slots. Newer 550MHz and 750MHz systems (and up) can use lower compression for the same number of channels and/or add additional channels or services.

If all channels are indeed "digital", that would mean a TV tuner could not be used to tune analog channels and a cable box would always be required.

[dkelly701]

If all channels are indeed "digital", that would mean a TV tuner could not be used to tune analog channels and a cable box would always be required.

This is what I have. It allows you to get all channels digitally if you have a box and still get the analog channels without the box.

It is from Time Warner's website.

Time Warner Cable’s Raleigh Division Completes First Full Market Deployment of Digital Simulcast
July 07, 2005

STAMFORD, CT – Time Warner Cable today announced its Raleigh, NC division has become the Company’s first cable system to digitally simulcast its analog channel lineup throughout its full service area. Time Warner Cable expects to complete its digital simulcast rollout in half of its 31 divisions by the end of the year. Digital Simulcast will ensure customers receive the best picture quality and the best sound on all of their channels.

“We have begun to aggressively deliver the digital simulcast of our analog channels, negating any perceived advantage of satellite delivered video service,” said Kevin Leddy, Time Warner Cable Senior Vice President, Strategy and Development.

The digital simulcast networks will be delivered to the same channel number as the analog networks so digital customers will still “tune” to the same channel as before. The simulcast signals will be converted into a digital format, positively impacting the image, sound and recordability. Current analog customers will not be affected as all analog channels will continue to be transmitted in analog.

“Our simulcast project provides consumers with the best of both worlds -- one hundred percent digital, which means crystal-clear picture and sound, as well as continued delivery of our analog service to those customers who prefer not to use a cable set top box,” said Tom Adams, Division President, Time Warner Cable of Raleigh.

Time Warner Cable expects to complete the digital simulcast roll out across its national footprint in 2006.

[dkelly701]

Back to the original question, if I record a digital signal to the built in DVR of my cable box and then play that recording to capture it, will it be an inferior capture than if I capture the "original" broadcast?

[Seeker47]

Upstream from the cable box (the coax from the pole) is a mix of 6 MHz analog channels (usually the two digit channels) and MPeg2 digital channels modulated into more 6MHz analog channels using QAM modulation.

Newer wider bandwidth cable systems include both analog and MPeg2 versions of the lower channels (hence the "all digital" hype). In reality, all digital information is carried over analog RF for cable or DBS.

Coax cable is fine for this purpose. The coax that comes out of a cable box is a low quality legacy single analog RF channel (3 or 4) just like a VCR.

If I follow you here correctly, then nothing matters signal-quality-wise until after the cable box, once the signal has been descrambled. In that case, and if it truly makes a visible difference, I should be making use of the S-Video output of the cable box, instead of the regular coax ? (There is a splitter in use, so that the TV can be fed independently, without having to rely on another device to get signal. This covers all but the premium and 3-digit channels, which do require the box.)

[edDV]

If all channels are indeed "digital", that would mean a TV tuner could not be used to tune analog channels and a cable box would always be required.

This is what I have. It allows you to get all channels digitally if you have a box and still get the analog channels without the box.

It is from Time Warner's website.

Time Warner Cable’s Raleigh Division Completes First Full Market Deployment of Digital Simulcast
July 07, 2005

STAMFORD, CT – Time Warner Cable today announced its Raleigh, NC division has become the Company’s first cable system to digitally simulcast its analog channel lineup throughout its full service area. Time Warner Cable expects to complete its digital simulcast rollout in half of its 31 divisions by the end of the year. Digital Simulcast will ensure customers receive the best picture quality and the best sound on all of their channels.

“We have begun to aggressively deliver the digital simulcast of our analog channels, negating any perceived advantage of satellite delivered video service,” said Kevin Leddy, Time Warner Cable Senior Vice President, Strategy and Development.

The digital simulcast networks will be delivered to the same channel number as the analog networks so digital customers will still “tune” to the same channel as before. The simulcast signals will be converted into a digital format, positively impacting the image, sound and recordability. Current analog customers will not be affected as all analog channels will continue to be transmitted in analog.

“Our simulcast project provides consumers with the best of both worlds -- one hundred percent digital, which means crystal-clear picture and sound, as well as continued delivery of our analog service to those customers who prefer not to use a cable set top box,” said Tom Adams, Division President, Time Warner Cable of Raleigh.

Time Warner Cable expects to complete the digital simulcast roll out across its national footprint in 2006.

That means you have a system with enough bandwidth that low channels are transmitted both as analog and compressed MPeg2 digital channels. It says nothing about bitrate of the "digital" channels which can range from poor to very good as determined locally by the cable company.

[edDV]

Back to the original question, if I record a digital signal to the built in DVR of my cable box and then play that recording to capture it, will it be an inferior capture than if I capture the "original" broadcast?

By definition it can't be any better. What you have on the DVR HDD is an encrypted MPeg2 capture. The DVR outputs are digital to analog conversions as follows:

Composite NTSC: OK for display, not optimal for digital capture unless you have a high quality capture card with 3D comb filter. Even if you do have one, the capture will be worse than the next best capture.

Component Y/C aka S-video: Digital MPeg2 is passed as Y and modulated subcarrier to the capture card. The capture card can directly redigitize Y and decode C to UV and then redigitize U and V. Quality is medium.

HD cable boxes will also have Y, Pr, Pb analog outputs but very expensive equipment (well over $1000) will be required to obtain better quality than S-Video and the difference will be very small.

If any of this is unclear, ask more.

...
will it be an inferior capture than if I capture the "original" broadcast?

Yes, but probably not by much.

The detail answer to this goes back to whether the DVR records the original MPeg2 off the sat (for "digital" cable channels) or whether it records at a lower bandwidth. Time Warner isn't telling us. It depends on local choice of compression for your cable system. If you ask any of these questions to the cable support number and even if you get to level 3 support they still won't answer.

[dkelly701]

Thanks edDV for your very detailed answer. I think I will look into getting new cables for my capture device.