Building my own DVR with Linux and Comcast

September 28, 2009 1:03 AM

I want to build my own Linux-based DVR. Need some help on the specs and interop with Comcast Digital Starter Cable.

Hi all,

I have decided to build my own Linux-based DVR to record and play the shows I get with Comcast Digital Starter cable package. I have read quite a few tutorials and all seems "easy" at first. However I would like to double-check with the hivemind on a few things.

My setup:

- Standard Dell destkop computer with 160 GB of Hard Drive and 2 GB of RAM
- Linux Ubuntu 9.04
- Comcast Digital Starter package with Motorola DCH70 set-top box. no HiDef channels.

1) I understand that I must buy a TV tuner card to receive the digital cable signal. Given my hw/sw setup, will this one do? Does it have a Linux driver?

2) Most tutorials and articles speak of "clear" vs "encrypted" cable signals. Why is that important? I "kind of" understood that encrypted = HD. However I don't have HD definition currently, and don't plan to get it. Will the TV tuner card mentioned above receive correctly the signal from Comcast?

3) The same articles say "plug the cable to your computer as you'd to your TV". However my cable doesn't plug directly into the TV. It goes first into the DCH70 box then there's another cable from there to the TV. Does that mean that I have to keep the set-top box and plug the cable going out of it (RF out) to the computer, then from the computer to the TV? Or should I insert the computer between the wall cable and the DCH70?

4) Using one of the standard DVR softwares for Linux (MythTV, Sage TV) will I be able to record more than one channel at the same time? How about watching a channel live and recording another at the same time?

I got excited about this DIY project and I feel like going through with it, but would like a little help from MeFi so I don't shell out some bucks just to find out that the setup doesn't work because Comcast blocks something or the setup is somehow incompatible with it.

posted by dcrocha to sports, hobbies, & recreation (10 comments total)

The big thing you are going to need is a CableCard from Comcast.

CableCard FAQ (Comcast)

However! It's not like they can just send you one in the mail. They have to (seriously) send out someone to install it. Yes, to put the card in.

However! It looks like, depending on what flavor/market of Comcast you have, you might be able to use a tuner like that, if Comcast in your area uses ClearQAM. (this is why I read AskMeFi-- to answer things and to learn things, and I think I've learned something today!).

Personally, I would get a tuner card that can TAKE a CableCard, but then try it first without it.
posted by gregvr at 2:04 AM on September 28

When you hear talk of clear verses encrypted digital cable channels they are talking about QAM most TVs nowadays can decode clear (unencrypted) QAM and many tv tuner cards can decode clear QAM. Cable companies are required to send some channels in the clear to their subscribers but typically they don't tell them which channel or sub channel they are on. the only likely way to get all of your digital channels is to connect the video and audio out of your set top box directly into your capture card and then use the dvr software to control the set-top box via an IR blaster. My understanding is that cable card probably won work well if at all in Linux. If you want to do this from a hobby standpoint to learn about all of this that's great but if you want something that will just work without hours of tweaking just get a DVR from Comcast.
posted by jmsta at 4:23 AM on September 28

The software project you are looking for is called MythTV, and specifically (since you mention ubuntu) you probably want the Mythbuntu distribution.

A list of compatible tuner cards can be found here. I personally used a Hauppauge WinTV PVR 350 for a few years in my mythbox, but that was pre-digital cable.
posted by namewithoutwords at 4:58 AM on September 28

So:

a) Encrypted channels are not necessarily HD, they are also channels that they bother to encrypt for whatever reason. On Time Warner in Brooklyn, for instance, I can get the networks, cable access, some shopping, Food Network and TNT "in the clear" everything else is encrypted. I'd definitely look into this before going down that route. That being said, you should still be able to get the analog channels (maybe).

b) You can get around this with a cablecard. The cablecard is basically the decrypting-only part of the set top box. Unfortunately, because of Big Legal Things, you can only get a cablecard in a Vista-Only peripheral in a PC with a special BIOS from the manufacturer. Here is some info on spoofing said BIOS.

Just so you're aware of the state of things, MythBuntu's april fools joke this year was to announce CableCard support. . . funny?

c) It sounds like you already have a cable box. I don't know anything about it, but I believe you can use what's called an "IR blaster" to have your computer control the cable box as if it were a remote. This way the computer would not be doing any of the actual tuning, it would just be recording the raw analog out from the set top box. You would only be able to record one stream at a time this way, as it would be whatever the cable box was playing.


Honestly, unless you have a good reason for it, or are bored, I've never really found a great reason for rolling your own DVR. But, I usually don't archive things myself (easier to download from usenet than to capture), so what do I know. Good luck!
posted by CharlesV42 at 6:04 AM on September 28

The other option, I believe, is to record the programs off the cable box via firewire. I don't remember if it works for encrypted QAM. I have done it with a Motorola box and a Windows PVR with some modifications, and it worked pretty well.

Another problem I constantly run into is that my cable company constantly changes the channels that the stations are on. So my PVR (BeyondTV) starts recording, but there is nothing on that channel. I then have to go through the setup process again. I love the PC-as-PVR, but that's a huge pain.
posted by gjc at 6:36 AM on September 28

your simplest path to victory:

1. install MythTV w/ Ubuntu or other compatible Linux Distro
2. acquire a Hauppauge HD-PVR and get around the requirement for a cablecard in your PC
3. use a firewire cable to change channels on your set top box

To gjc's point above regarding recording via firewire, when I was an RCN customer, I was able to hookup a firewire cable to capture the digital transport file for all of my channels as RCN didn't 5c encrypt their firewire ports at all. So, I was all about having digitally perfect HD copies of Return of The Jedi and The Prestige recorded off of Cinemax or Showtime. It is still, to my mind, the best way to go if you have a provider who is charitable enough to leave this hole open in their distribution channels.

Both Verizon FIOS and Comcast encrypt their firewire ports, so now I'm dealing with the pale shadow that is the Hauppauge HD-PVR and its associated flakiness. In my experience, the HD-PVR has had about a 90% success rate with recording stuff and not losing anything or crapping out in the middle of a broadcast. The 10% of stuff is tolerable because I can just re-record the broadcast, but I imagine that if I was a sports fan or had some other demand for recording live events, I'd be pretty annoyed by it over time.
posted by bl1nk at 8:57 AM on September 28

bl1nk has it, but I don't think firewire will work to change channels. Not a problem since the Hauppauge has an IR blaster that will.
posted by togdon at 12:21 PM on September 28

You should definitely head over to the SageTV forums (http://forums.sagetv.com). I have used SageTV (on Windows) for years and it's a great product. Not free or open source, but well worth the money at $80.

To record more than one program, you'll need more than one tuner. If you want both tuners to have the same lineup, and you have cable boxes, you'll need to rent more than one cable box. For changing the channels via IR to the cable box, I recommend the USB-UIRT.

Sage has some great benefits:
- low power extenders boxes (so that you don't have to put a computer at each viewing location, although you could if you want to)
- integrated video, music, and photo libraries
- a strong, helpful forum
- solid support (although email only)
- many user-developed plugins

They have a 2 week trial. Give it a try!
posted by reddot at 8:18 PM on September 28

I am currently using the firewire cable to change channels on my Verizon FIOS set-top box with an HTPC that has been running for four years, so step 3 isn't some piece of advice that I pulled out of vicarious trawling of forums.

I'd also give a huge thumbs up to Sage.TV, they're a great option if you want something that just works and aren't particularly interested in tinkering (though there is still a lot to tinker with on Sage)
posted by bl1nk at 8:38 AM on September 29

Sorry, but folks saying you can't use a cable card with a PC are simply wrong. We have that set up, and it works. Yes, they seem to change the channel set-up, and that IS a pain. But we just got around to implementing HD, and it's good. The HD is decoded in the new graphics card.

With our cable setup, you can sometimes record 2 different things at once. It has to do with the tuning. Apparently, there are multiple channels for a given tuning. If both things you want are from related tunings, then you're good. (I don't handle the technical stuff on the PVR, it bores me. The other half enjoys it).

We're running Mythbuntu on a dedicated box, configured specifically for compatibility with things Linux, and being quiet. This is our second MythTV box, and our second configuration when we moved from South Africa to Switzerland. In South Africa, the system worked like a charm, but we didn't have the computer doing the tuning. Here, it seems to be a constant fight, but maybe now it will settle down (until the next time they remap the tunings).

Oh, to be fair, our arrangement is extra complicated because we are only interested in English language material, and so our main channels are from the UK, so we have to get schedule information from multiple sources (Swiss channels frequently have an English audio choice). Frankly, the local cable company seems seriously amateurish to us.
posted by Goofyy at 10:01 AM on September 29