Sunday, March 1, 2009

Fixing WEC Le Mans

A guy on tho local arcade forum bought a standup driving arcade cabinet, with the 1986, totally upstaged by Outrun, driving game Wec Le Mans.

Trouble is didnae work and as he was local (well 80 miles away - that's local in these parts) I said I would take a look if he dropped it over. Huge board, 2 boards in fact in a stack.

Fault - BAD RAM 7G & 8G

Cause - out of spec 74LS157 @ 10F

Didn't think the death of 2 RAM chips at once was likely, went round the controlling logic with the probe and everything looked fine. Resorted to using my HP Logic Comparator on the logic driving the RAM and it showed that all the 157s outputs were taking too long to change, most of the time the chip was inline with the reference chip, but on the transitions I got alarms on all 4 outputs. Really should use that comparator more, it came with a stack of little PCB cards to set up for common chips so you can just plug the test board in for a range of chips. Never got round to making them up so have to faff with dipswitches every time.

Once the RAM was fixed I could get a good look at the stripe of graphics, from the previous photos I had assumed that it was the good stripe and all other stripes were not being drawn. I now got the impression that this stripe was ALL the graphics compressed as there were little squashed cars whizzing about and bridges coming and going.

While I had the comparator set up for LS157s I went over the board and found that 13A and 14A were not great, replacing these resulted in the game track re-appearing.

Went round the board interrupting the clock to 74LS273 chips to see which bit of the board was doing what. When I found the area that was drawing the stripe I went round the nearby chips and found an LS161 that had all its outputs stuck high, as soon as I started touching the pins on this chip the stripe flipped from the middle, to the far right, and then back to the far left. Replacing that chip fixed all that and ....

Now the game looks like its 100% but its not quite, the track kerb has no stripes, sometimes the grass doesn't either, the score board is a bit messed up and the sprite masking for where the countdown number is over the clouds doesn't always work.

Also found a couple of very dead 74LS273s but replacing them hasn't done anything obvious, and also swapped a few 161s that were showing up as a bit flaky, again nothing obvious but that's 5 fewer F chips to die later.

Put in good few more hours on this board this morning and I am 90% sure I know what the fault is regarding the missing kerb stripes - an LS166 at 11G on the CPU board with a stuck output pin (13 Q). If I inject rubbish data on this pin and wake up the downstream cct I get jaggies on the track edge where the stripes would be.

Trouble is LS166s aint common, and I have been through my scrap drawer twice and I have none.

Fixed the problem with the clouds and the score card too, problem was a sick looking LS273 at 27I on the video board. I initially thought there was a problem with the masking of the countdown timer over the clouds, but it struck me that the clouds were always grey and the mask changed depending on the track that was playing in the demo. Loaded it up in MAME and the clouds on the sunny day should be white, the ones at dusk have a bluey tinge and the ones at night are black. If I interrupted the clock to the 273 at 27I I could get some of the clouds to change to white. Its outputs were all alive but they looked sick, replacing this chip has fixed all the remaining issues on the board (bar the one that I think is the LS166 as above).

Took the old 273 out and tested it, man I love it when a suspect chip ...


With a new LS273 I now get

1) the correct clouds per level, they look better too.

2) horizon detail, trees, buildings, city skyline.

3) the correct display of the score card. The colours are now right and the banner on the side says Le Mans 24, rather than "Le ManLe ManLe ManLe" in dark grey text on a dark background.

Found an LS166 on my now-scrapped TMNT board, fitted it and the centre line vanished, arse, one step forward, 2 steps back.

Looking back at the old photos its clear that the centre markings was just a solid line which is wrong, just I didn't notice it before. Comparing with MAME there shouldn't even be a single centre line, it's a 3 lane road, and the kerb striping is instep with the road markings, so its likely to be a single remaining fault that's causing both issues now.

I took the new 166 back off to be certain that the loss of the centre line wasn't just another fault appearing. When the old chip was back in the board the solid line is back. At least its not going flaky on me.

Hmm - still on the trail, who said fixing boards was quick and easy ;)

Finally - traced the fault through to an LS151 at 10E, its output was weak and died totally after a couple of touches with the logic probe. Slapped it in my tester and..., thank fook for that!

Replacing this chip brought back the correct road markings, the road stripes and the kerb stripes.

She's perfect again, soak tested it for 3 hours and it didnt miss a beat. Lacquered up the reworked sections of the board and my work is done.

Monday, January 26, 2009

The Great HDMI Ripoff

Cabling has always been a huge cash cow for audio, and more recently AV, vendors, the theory goes that when people drop a large amount of coin on a system you can usually convince them to spend between 10-20% of the total on cables, especially if you sow the seed of doubt in their mind that without buying quality cables they won’t be getting the very best out of their new system.

Meanwhile back in the real world the laws of physics have an annoying way of making the above sales pitch rather meaningless, slightly meaningless in the analogue world, and completely meaningless in the digital world. This scam has been going on for as long as audio gear has involved electrical signals and it shows no sign of going away anytime soon.

Back in the early 90s when I was into hifi the 10%-20% rule was alive and well when it came to speaker cable and system interconnects. If you had spent 10K on a system and were happy to part with 1K for under 20 foot of wire then there were plenty of vendors happy to take your money. For your money you got what were certainly very very aesthetically pleasing cables and pages and pages of pseudo science that proved what a wise investment you had made. The laws of physics on the other hand said that as long as the wire was thick enough and short enough the difference between cable that costs 200 pounds a meter (yes they exist) and a cable that cost 20p a meter would be virtually impossible to detect, even with extremely high grade measuring equipment. The distance issue is the important one, if you are planning to roll out 20km of the stuff then yes you will see signal degradation (due to resistance) and attenuation (due to other signals nearby interfering with your signal, especially if you have multiple cables lying side by side). If you plan to run 10 meters of the stuff then the resistance will be utterly negligible and unless you run it next to something very very noisy (signal wise) then the attenuation will be undetectable, unless you have very very sensitive equipment. The other issue, that of the cable being thick enough is not a major problem, using telephone wire (think CAT5) for speakers is not a good idea, it really is too thin to hang speakers on the end of, something along the lines of the wire builders bury in walls to carry mains power to the sockets is about right, so about 3-4mm of copper in the core. So instead of 7p-a-meter phone wire you should use 9p-a-meter slightly thicker wire.

How much you were willing to pay for wire that looked pretty on the outside was your own concern. The same applied to interconnects, except here the issue was even more of a none event, cables the connect hifi components together are often only 1M long, often shorter still, so the signal is not going to be damaged as much as the signal on the speaker wire might be, so around the order of four fifths of fookall. The benefit of shielding was often bandied about, interconnect wire is usually shielded, speaker wire usually isn’t, this usually is not a problem either for short runs.

Fast forward to today, and the fattest juiciest cash cows in the market are the HDMI cables, those chunky cables that carry the audio and video digital signals from your high end output device PS3/BluRay/HD Recorder to your telly. Pop into any shop selling TVs and they will have a range of cables not too dissimilar to the range I was presented with the other day. Low end cable was $50 (20 quid), mid range was $90 (37 quid) and the best they had was $200 (65quid). I was buying a nice big TV and was seeing what the sales monkey would do to the price if I was to buy a BR player as well, after much muttering he came back and said he could throw in a free HDMI cable worth $50 apparently. Having not done my homework on BR players and not being greatly interested in getting a free $50 cable that was ten times overpriced anyway I declined and then went and purchased the TV somewhere else.

This blurb the sales droid came out with was that the pricier cables allowed the colours to be purer, more data could flow through so the picture was better and kept out nasty interference better. All fine and dandy concerns IF the signal was analogue AND you were running 10KM of it. In the case of HDMI neither apply, even the audio component of the signal is digital.

Being digital the signal either arrives in a state that can be decoded perfectly, or it is so badly stuffed that it’s next to useless. This is why when you have a lightning strike nearby and your digital TV loses its signal, its coughs and farts about with a blank screen (or a frozen screen) and then comes back with a perfect picture again. Analogue signals give a degraded picture with all sorts of crap on it for the duration of the RF blast as the signal gets messed up, digital signals can take a huge amount of abuse before they get messed up enough that the receiving end cant decode a perfect data stream from it.

N.B. the artifacts you see on some digital channels are to do with compression, the transmitter is sending a lower grade picture so they can cram more channels into their allotted bandwith slot. 1080p free to air broadcast TV pictures can be truly stunning, one of the channels here does it properly and the image is like looking through a window into the studio, it is amazing. Another channel transmits at 1080p but the image is not so great, both come from the same transmitter and arrive at my house via exactly the same path, the GIGO rule (garbage in garbage out) also still stands in this day and age.

Add to this the fact that for a cable carrying digital signals over such short distances to suffer interference extreme enough to flip bits you would have to be running it next to something like a arc welding equipment. If your analogue cables are not introducing immense amounts of hum into their signals then your digital link is going to be nigh-on perfect, it will either work perfectly all the time, or not work at all, in which case the cable is faulty, not "too cheap".

There is no way a low grade cable can impact the colours you see, or the quality of the sound, to do so you would have to decode the signal and then process it to look worse and then re-encode it perfectly. This requires a degree of technical wizardy that plain old signal noise cannot achieve, its the equivalent of you speaking english to someone in a very very noisy bar and for the background noise to alter what you are saying so that the other person hears you speaking fluent and coherant french. The received signal still has to make perfect sense protocol-wise or the receiver will reject it as a valid signal, for the payload to be changed but not the protocol bits is about as likely as the bar translation example above.

Jitter? Don't even try me on that one!!

Which is why I have bought the cheapest "pretty" cable I could find on eBay, pretty because I like pretty but mainly because if a cable looks well made it probably is, bare generic black ones might be fine but their auctions didn’t have any up close and dirty pictures of the cables so it was hard to tell.

The price I paid for a 2M cable? AU$2.99 (about 1 pound 35p). All up with shipping from Hong Kong to my door it was (AU$9.98) a nats over 4 quid.

To move a digital cable a mere 2 meters, without any industrial welding gear nearby I would reckon you could put the signal over wet string and get away with it.

I shall invest the money saved on curry and ale this Thursday night and shall politely inform the sales droid if and when I buy a BR player that I already have a cable that is as good as his finest.

When will I buy a BR player? Hard to tell, if and when I do it will be an impulse buy and the urge to play with it that very day will be immense. Buying a cable ahead of time means I can buy without fearing I will have to sign up to the bend-me-over-and-sell-me-a-$50-cable club just so I can hook up the player as soon as I get home. Having to wait a week might well have caused me to drop the $50 just to get instant gratification. Even if I don't get a BR player, HDMI as a connection type is not going away anytime soon, eventually I will need a cable, and there will still be enough loonies out there to make selling $5 cables for $50 to $200 a very common practice.