Category: Technical

There are certain aspects of my mythtv PVR system I’ve been unhappy with. They are:

• Low hum on all recordings. “hoommmmmmmmmmmmmmm”
• Bad video output — the display flickers and you can see pulsating waves

Hums are usually a sign of an Audio Ground Loop. If two electronic components are at slightly different potentials, usually because they are plugged in to different sockets, an audio cable will actually create a complete circuit. A small amount of charge flows between the two components, and you hear a hum out of your speakers. Radio Shack makes a cheap doohickey called a Ground Loop Isolator that solves the problem.

Unfortunately whenever I tried using the Ground Loop Isolator, I would hear a different, even more annoying squeal. So for a while I lived with the hum. Today, after some experimentation, I discovered the the squeal occurs because the onboard sound jacks are close to the monitor plug. If I disconnected the monitor cable, the squeal would change! So I bought a 24$ sound card on clearance, positioned it far away from all the other jacks, and now there are no annoying noises coming out of the speakers.

As for the video problems, my PC->TV converter was the cheapest possible model. The next version up from the same company solved all the problems. The one I have now is a Grandtec (like my old one) but it has a little remote, supports more resolutions, and outputs to component as well as svideo. It seems scan converters is one place where it does no good to get the cheapest option (unlike 24$ sound cards).

After a couple months of playing around with mythtv, I’d qualify the experiment as a total success. It records shows perfectly (ok, occasionally misses the first 15 seconds), plays them back, manages them, etc. I can extract the shows, convert them, burn them if I want.

In my last entry, I mentioned the only remaining problems are slow channel changing and occasional crashes. The only other problem I’ve had is frame jitter, caused by the difficulty of converting progressive vga to interlaced ntsc.

Well the channel changing is always going to be slow, but I’ve been keeping up with the development version of mythtv, and it almost never crashes now. Rarely the frontend locks up, and almost never does the backend die. And now there’s experimental code to try to reduce the frame jitter, and it works really well. I call it the CNN Test — is the crawl at the bottom of cnn smooth or shaky? With the new code it’s not perfect, but it’s a lot better than it used to be. It’s much easier to forget I’m watching output from a computer now.

The mythtv team writes some amazingly robust, quality code. The install process, however, is still ridiculously hard for anyone other than an experienced linux guru. There’s not even a configure script that tells you if you have all the pieces installed! Take a look at the 2000 page install script sometime, you’ll see. Naturally video capture is a delicate art, so there’s a lot of ground to cover, but the actual software installation of mythtv could be streamlined greatly. Hopefully one of the TODO items for 1.0 will be some sort of easy installation.

Of course it wouldn’t have been any fun if it hadn’t been challenging. Henry likes to complain about my computers, he has since I’ve known him. This is not his fault, I just tend to do weird things with my machines that make them behave differently than most people’s machines — therefore, they are unfamiliar and weird. Mythtv, however, is a very smooth, invisible system. I don’t think he can complain about much of anything (except the channel-change speed — that man weilds a remote like an automatic pistol).

Last gripe: mythtv doesn’t stop recording if the disk is full. this causes big problems and crashes. come on people.
this has been fixed in newer versions.

pps: The writer of the Tivo Hacks book recently advertised his services to build mythtv machines for 1200$. Given my experiences, that is way more money than necessary. I’ll do it for 750$.

I have now used my mad skillz to use two infrared devices at once. This may seem arbitrary, but what it means is that I no longer have to point the remote directly at the receiver in order to change channels. It now works clear across and into the kitchen with no trouble (except that then the text on the tv becomes hard to read at that distance).

The only functionality still missing is reencoding shows to a format I can watch later or burn to a cd, but I’m fairly sure that will come around. For now the system is at a point that I feel I can recommend it to friends, as long as they keep in mind a couple things:

• It’s still development software, and it does crash from time to time. recovering from a crash involves typing on a keyboard, but it’s not that bad
• Channel changing is slow, about 3 or 4 seconds. This doesn’t matter if you’re just watching recorded shows, but sucks if you plan to surf channels.

The replacement ram for my tivo machine finally arrived. When I first put that machine together, it crashed and crashed and I could not install the operating system. Now with this ram I can put it together again and see if the problem has been solved.

I have this sick feeling in my stomach that, in fact, the problem has not been solved and this fucking thing is still going to crash. Please pledge your support by replying to this and wishing me luck!
Continue reading “Finally!”

I’ve made some good progress in getting the Tivo machine to work. Once again the gory details are in the extended entry to spare the people who expect pictures of cats.
Continue reading “good tivo progress”

After a lot of headaches and trying to get various parts to work, I have to alter the pricelist a bit.
Continue reading “Parts parts parts”

For the benefit of no one but those who randomly google for this information, here is the complete listing of hardware I used to building my tivo-style computer. The goals of this computer:

• Display tv channels transparently (no weird interaction, just like a cable box)
• Act as networkable mp3 jukebox
• Record tv shows, including those on digital cable
• Basic pause, rewind, commercial skip
• Near-silent operation

The complete list is in the extended entry below.

Continue reading “Building a PVR”