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Digital TV on Linux = aaargghh!!!

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A blogged previously, over the holidays I built a Linux-based PVR with the anticipation of recording TV when I’m out and watching the programs (mainly documentaries) at my convenience.

The computer system side of the equation worked very well, but the quality of digital TV reception I’ve experienced is driving me up the wall.

Let me chronicle the events leading up to my present state of extreme frustration.

– I used an ASUS barebones machine for the PVR itself. Simply buy the small form-factor barebones system and add your choice of AMD CPU, memory and SATA disk.

– I installed Mythbuntu on the new machine. Worked like a charm with all components supported perfectly.

– Acquire TV tuner card and add to the barebones. I can happily report that both the Hauppauge WinTV Nova-T 500 PCI and the Leadtek Winfast dtv1000 work fine with recent Linux kernels, and hence distributions.

– Get a StreamZap. Pricey but works out of the box with Mythbuntu and can be used for other apps in addition to MythTV.

Then came the moment of truth – testing of the completed PVR at my parent’s place on the NSW Central Coast.

– I plugged the DVB-T antenna cable into the TV tuner card and configured the channels by doing a full scan.

– MythTV “scanned” all available channels so this proved it “worked” in the sense that all the drivers and app are configured correctly.

– Then, in the MythTV front-end I proceeded to simply watch one of the scanned channels and it would only “lock” onto NBN Digital and SBS. When attempting to view any other channel, MythTV could only get a “partial lock” and they would not display.

Wanting to determine whether I was facing a reception problem (there was no reason to believe it was the PVR as two channels did display), I brought the unit into the office in St Leonards and set it up there.

EUREKA!!!!

I was able to scan and tune into, or “lock” onto in MythTV’s parlance, a whole swag of channels, including some from WIN TV in Woollongong. The only channel missing was channel 7 in Sydney despite being about equidistant from the Sydney CBD and Epping (or whereever it broadcasts from). This wasn’t such a big deal as I could get Prime perfectly which is essentially a re-broadcast of 7 in regional areas around the state. The biggest problem is the office is not the ideal place to sit back and watch Penn & Teller on a Sunday evening.

So, having come this far, I figured testing it at my Parramatta apartment wouldn’t hurt either. At least that would give me some idea of any reception difference between metropolitan Sydney and the Central Coast.

I cart it up three flights of stairs, fire it up, re-tune it, and hey, presto, everything except channel 10 despite a “100%” signal strength reading!

Okay, think calm blue ocean, calm blue ocean.

I think now is the best time to add that built-in digital TV tuners in all three locations work as expected. The digital TV on the central coast picks up all channels (Sydney and Newcastle) with ease. We duh, it would be pretty big paperweight it if didn’t, but I’m sure you can gather what my next point is going to be.

The DVB-T signal strength in all three locations is more than adequate so it is a problem with the PC tuner’s reception capability and/or the software that controls it in Linux.

After reading through the DTV Forum, I discovered there are a multitude of factors that determine whether you can receive (and display) digital TV in this country. Understandable, but I just wish it wasn’t this difficult!

Anyways, one suggestion – scoffed at by many – is to use a small signal amplifier to boost a weak signal into a TV or, in my case, a TV tuner card. So I gave it a go yesterday and it did result in a slight improvement. The PVR can now display channels Nine, Seven and Ten. Of course, the only two channels worth watching – ABC and SBS – don’t display. Do I remember saying SBS did work without any amplifier? Ah well.

So I’ve come the following conclusions:

1. PC TV tuner cards are nowhere near as tolerant of weak signals than proprietary tuners built into televisions.

2. If 1. is not true, the Linux drivers for PC TV tuner cards are not as tolerant as their Windows counterparts. Yes, I could install Windows on the PVR and try the proprietary drivers but that would negate the other benefits I get from having a Linux-based PVR. Hey, it can double as a home server!

3. Sure, I may have incorrectly configured, or unintentionally omitted, something from the installation and setup process, but that doesn’t explain why some channels display perfectly while others don’t display at all. If anyone has any ideas I’d love to hear them! Your input will also help other Aussie MythTV users.

4. The black art of MythTV’s “scanning” and “signal strength” don’t seem to correlate very well into some sort of predictable trend. For examples, a scanned channel won’t necessarily display, low percentage signal strengths can produce a perfect display, and a 100% signal strength won’t necessarily produce a display at all. Go figure!

My next step will be to try another, more powerful, type of amplifier. Hey, it’s been the most promising result so far! I may also call a local antenna guru to see if there’s anything else that can be done to improve signal reception.

Failing everything, I think I’ll re-purpose the machine as a desktop and look for a black-box PVR that does what it’s supposed to do 🙁

6 Comments

  1. Back in the day, you could find several transmission problems with analogue TV. It was possible to have bad signal strength resulting in snow, interference resulting in periodic noise (perhaps at the same point on the screen all the time, if it was 50Hz correlated noise) and it was possible to have ghosting, where you get multiple paths from reflected objects like trees.

    Those problems still exist, except that you just can’t isolate them because the picture is all or nothing.

    The signal strength metre will tell you that you have enough signal, but it won’t tell you that you have too much interference or ghosting, which is just scrambling the data. Hence why my hardware tuner also has a “quality” metre, which isn’t always correlated to the signal strength metre.

  2. When you’re scanning the channels under Linux, is it coming up with the correct frequencies?

    I had an issue for ages with SBS, here in Melbourne. All the other channels worked absolutely perfectly, and my TV’s own digital tuner had no problem with SBS, but my Dvico Fusion Dual Digital 4 card simply would not receive SBS correctly.

    It was only after a month or so that I remembered that when I first tested the card in a different PC under Windows, SBS had worked perfectly that I decided to look carefully at the tzap.conf file that had been generated, and I found it had been coming up with the wrong frequency for SBS every time I scanned it.

    After I forced it to use the correct frequency for SBS here, it’s been working beautifully ever since.

  3. I have a mythtv setup in Newcastle and I only have around 12-15% signal on most of my channels, but it still locks. I am also using chained dvt1000 cards and I don’t lose signal along the way either so it seems to be working fine for me. Next step is to network the house for access in each room.

  4. Thanks for the pointers. I’m still playing with it, but I’m left wondering how to fine tune a channel’s frequency.

  5. Good news! Looks like I have cracked the problem by upgrading the software. Will blog about it soon.

  6. Im keen to hear how. I’m in Brisbane with MythTV on mythbuntu, the Hauppauge WinTV Nova-T 500 PCI card in a MSI Media Live box. Im having problems with ABC and SBS Brisbane but all other channels are fine. The weird thing is that ABC gets the highest signal strength of about 55% but its pixelated has dropouts and sound crackles. Im not sure what to do next. ABC is the only channel I watch so for me it makes the entire system almost useless.

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