Film-Tech Forum ARCHIVE: CP500 Output Distortion and Panel Meter Glitches

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Posted by Tom Ostertag (Member # 8318) on 09-03-2016, 10:28 PM:

Hi all,

My CP500 has developed an odd intermittent problem. Occasionally one channel of audio output will sound distorted after power-up. This sounds like it's happening in the digital domain, almost as if it's missing one or more bits of precision, very similar to a "bit crusher" effect. The issue is present with any input selected. The first time I heard the issue it was in my front left channel, and most recently it happened in the right surround. In both cases, power-cycling the processor solves the problem.

To make matters worse, when I power-cycled the processor after the last occurrence of this issue, my front panel meters started showing periodic high level spikes on the Rs and SW channels (even with a silent source), while the outputs sounded fine with no clicks/pops/distortion. While passing audio, the meters also started frequently freezing for several seconds at a time before updating again. This processor has always had some sort of "phantom level" on the SW meter since I acquired it, but this is much worse.

I'd have to verify, but I think I'm on the latest software revision. Also, the clock stability modification has already been performed from the factory.

Has anyone else noticed instability in these areas? Should I start looking at capacitors, or any specific boards? Component-level troubleshooting and repairs are not foreign to me, but any ideas on where to start would be much appreciated!

—Tom

Posted by Marco Giustini (Member # 4544) on 09-05-2016, 11:08 AM:

Hi Tom

The vu meters freezing and the vu meters spiking with no sound is something I've seen before and AFAIK it's a known issue and you don't have to worry about that.

Please make sure you are on the latest SW which is 1.65.

Regarding the distortion, that is a known issue too but I am not sure whether it's ever been solved or not. The 500 had tons of known issues - some of those could be fixed by reworking, others by replacing components, others by replacing boards. I cannot honestly remember if the distortion issue was a fixable one, I'm sure others will be more helpful than me on this subject!

Posted by Sean McKinnon (Member # 612) on 09-05-2016, 04:21 PM:

I'm just curious how long you've had this CP500 and if it was working without this distortion and display issues at one point?

Posted by Tom Ostertag (Member # 8318) on 09-06-2016, 12:50 AM:

Thanks for the info so far.

I've had this processor for probably a little less than a year. I got it off eBay, so its history is unknown. I've added the Cat 885 6-channel ADC and 683 crossover cards to suit my application in my home screening room, and also replaced the 5V 1F "super capacitor" on the Cat 984, which was physically leaking a bit. Apart from that, the components and PCBs look pristine.

There has always been some noise in the VU meter for the SW channel, with the meter randomly rising and falling without any signal I/O. I mentioned that to one of my friends, and he didn't seem at all surprised, so I figured it was a common and harmless issue. Since it got worse around the last occurence of audible distortion, I thought there might be some correlation. I've been a victim of troubleshooting coincidence enough times to know better though, so I realize they may not be related.

Since I've had the processor, and I use it a few times a week at least, I've heard audible distortion as I described only twice. Probably a couple few apart, with the most recent one being last Tuesday. In both cases, a power cycle eliminated the symptoms.

Signal flow would suggest the source of the distortion is either the Cat 675A DSP that's used for equalization, or the 684 that's telling it what to be and do. Of course the components look flawless, but I haven't tested any electrolytic capacitors yet. This may just be one of those little issues that has no reasonable explanation.

I work in broadcast television, and we have broadcast-quality HD video monitors that occasionally crash and have to be rebooted. Monitors! The days of digital device problems surprising me are long gone.

Well after all that, I just powered up my CP500 to check the software version, and I'm on 1.61. I thought for sure I was on 1.65. That's a real glaring issue right there!

More to follow when I get the thing updated, hopefully later this week.

Posted by Marco Giustini (Member # 4544) on 09-06-2016, 03:41 AM:

Hi,

Get rid of 1.61 ASAP, that's evil [Smile]

The subwoofer VU meter spiking at random is definitely "normal", I'm confident it's not related to the distortion you have heard.

I love the CP500 but it's not the best product made by Dolby. I'll see if I can find what was causing the distortion and see if there was supposed to be a solution.

Posted by Mark Gulbrandsen (Member # 72) on 09-06-2016, 02:41 PM:

Hahaha! I'll never forget the CP-500 firmware version that caused the 500 to factory default. I was told by reliable sources at Dolby that it was written in the code to do this after so many power cycles by a disgruntled Dolby Employee. I think it was 1.28 or 1.32, or 1.46 or something like that. Dolby really pissed off a lot of new customers with that issue.

Mark

Posted by Scott Norwood (Member # 30) on 09-06-2016, 03:12 PM:

What format is selected when you have this problem? We had a similar issue in a 6-plex, but only in SRD. Switching to an optical format (and leaving it there) or rebooting fixed it.

We never actually resolved this one, but it stopped happening when we stopped moving the SRD cards around from processor to processor (they had six screens. SRD readers in every projector, and six CP500s, but only three sets of SRD cards). The theory at the time was that it may have been a backplane issue. They were also running version 1.61 firmware, as it happens. If I remember correctly, this only happened on the first show of the day (when it did happen, which was only occasionally).

Posted by Marco Giustini (Member # 4544) on 09-06-2016, 04:10 PM:

I experienced that too and my 500's were running 1.61 too.

Mark: [Eek!]

Posted by Tom Ostertag (Member # 8318) on 09-07-2016, 01:53 AM:

quote: Scott Norwood
What format is selected when you have this problem? We had a similar issue in a 6-plex, but only in SRD. Switching to an optical format (and leaving it there) or rebooting fixed it.

When it does happen, it's on that output/speaker channel, no matter what the input is. You can switch around between external 6-channel, nonsync, SR, etc, and it's there. I don't have a Dolby Digital penthouse yet, so I don't know about SRD. Interesting that you had distortion somewhere else, and reboots fixed it.

Well, I finished the upgrade to 1.65 tonight. Super easy with Load500 v2. The hardest part was lugging the XP desktop with the real serial ports upstairs!

So far so good! I rebooted the CP500 a couple times, and haven't seen bad meter readings yet. No distortion yet either. Only time will tell if that was all it needed!

I was skimming through my last post, and realized I called the Cat 685 an 885, and the 684 a 984. That's what I get for late-night typing. Oh well, I'm sure y'all knew what I meant.

I'll be doing some more screenings soon, so I'll keep an eye and ear on things closely and report my findings!

Posted by Frank Bolkovac (Member # 6201) on 09-07-2016, 11:48 AM:

Mine did the same with no effect on the sound. Some spiked, some didn't. Four different multi-plexes. All 500 processors. If your getting distortion from time to time, it might be coming from someplace else. Lots to check after or before the processor.

Posted by Ken Lackner (Member # 1002) on 09-07-2016, 12:50 PM:

quote:
Well, I finished the upgrade to 1.65 tonight. Super easy with Load500 v2. The hardest part was lugging the XP desktop with the real serial ports upstairs!

A Keyspan USB-to-RS232 adapter has never given me the slightest bit of trouble. I've used them on multiple laptops running both XP and Windows 7.

https://www.amazon.com/s/?ie=UTF8&keywords=tripp-lite+keyspan+usa-19hs&tag=googhydr-20&index=aps&hvadid=78443082048&hvpos=1t2&hvnetw=g&hvrand=14416756660006450799&hvpone=&hvptwo=&hvqmt=b&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1013962&hvtargid=kwd-75246 206808&ref=pd_sl_5vdlri4ehb_b

Posted by Marco Giustini (Member # 4544) on 09-07-2016, 04:17 PM:

Frank,
The noise coming from some of the speakers was a known issue with the 500's. Probably your processors were not affected (it may be that there was something that could be done to remove the issue) but if someone reports of distorted sound which fixes itself with a reboot, it's most likely the 500 itself!

I have found a reference to an EFB 188 regarding improper noise and tones but unfortunately my notes do not specify what the EFB 188 is!

Posted by Louis Bornwasser (Member # 3063) on 09-07-2016, 04:51 PM:

Don't forget the small jumper wire from case ground to low side of an output. Kinda not in all manuals.

Posted by Tom Ostertag (Member # 8318) on 09-08-2016, 12:32 AM:

quote:
if someone reports of distorted sound which fixes itself with a reboot, it's most likely the 500 itself!

Agreed! I've most certainly narrowed it down to the processor. I have some audio distribution amps in another frame, and a patch panel, but the reboots and my booth monitor rule out those things. The distortion was verified to be present immediately after the CP500, and not before. And the CP500 was the only device I rebooted to clear the issue. Well, the amps were powered down too for safety, but the booth monitor shows the distortion before the amps.

quote: Louis Bornwasser
Don't forget the small jumper wire from case ground to low side of an output. Kinda not in all manuals.

Indeed, the chassis–signal ground jumper is in place on the CP500, and the CP500 is used as the central ground for all line-level devices. Not a ground loop in sight!

I ran a movie through it tonight (via external 6-channel in this case), and I didn't have any audio issues. Also tried some non-sync sources with the same result. However, the phantom meter levels came back, so that looks like a minor annoyance I'll be stuck with. I expected that, after all the reports of that being normal for others. As long as it's nothing in the audio! More monitoring to come.

Posted by Tom Ostertag (Member # 8318) on 09-23-2016, 07:46 PM:

The audio distortion problem just happened again, this time in the center channel. So updating the software version wasn't the cause, though I am happy to be on 1.65 now. Since the issue happens on power up, I'm going to try turning on and off the CP500 with its power/bypass button, rather than the rack power distribution. Maybe the electrical noise from starting the other line-level equipment at the same time was causing problems. I always turn the power amps on after the Dolby/DTS/booth monitoring gear, so there shouldn't be any major power ripple from those. We'll see!

Posted by Gordon McLeod (Member # 33) on 09-23-2016, 08:08 PM:

Did you use star washers on the mounting screws to the rack?

Posted by Marco Giustini (Member # 4544) on 09-24-2016, 04:56 AM:

As I said there were a number of technical bulletins for the 500. To be honest the 500 has always been infamous for not being the most reliable piece of equipment - in term of those little glitches you experienced.
Others may have more experience than me but if this happens every now and then I wouldn't be too much concerned.
There may be a fix for your very problem but I am not sure, I know that my 500's were suffering the same issue but very rarely.

Posted by Tom Ostertag (Member # 8318) on 09-25-2016, 03:46 AM:

quote: Gordon McLeod
Did you use star washers on the mounting screws to the rack?

Hmm, I did not. I was under the impression they only recommended those to avoid ground loop-related audio issues like hum, buzz, etc., but now I see the note in the manual about avoiding "electrical noise" as well, so it's probably worth a shot! I am, however, using a round-hole rack with clip nuts, so the ground connection through the rack screws to the rack itself isn't exactly guaranteed. Perhaps a ground wire from the CP500 chassis bolted to the rack frame would work better.

quote: Marco Giustini
...if this happens every now and then I wouldn't be too much concerned.

That's what I'm starting to accept. It would be great to not have to worry about it though. It seems like the issue only happens when I'm trying to show off my system and impress guests, but isn't that always the way?

Oh well, it's no big deal. If I come up with something that magically works, I'll let you all know!

Posted by Mark Gulbrandsen (Member # 72) on 09-25-2016, 09:56 AM:

Are you sure the distortion isn't that channel of the power amp failing? I still see this happen way more often than actual processor failures.

Mark

Posted by Tom Ostertag (Member # 8318) on 09-26-2016, 02:57 AM:

That was one of my suspicions after I heard the distortion for the first time. Well, after I ruled out a blown HF compression driver, which it really sounded like at first.

But no, the issue is audible quite clearly from my MS-100 booth monitor, while monitoring processor outputs. When preparing to power-cycle the CP500, I do shut down the amps first, and the distortion persists from the booth monitor until the CP500 is rebooted. All is good after that. The distortion has happened on L, Rs, and C channels in that order, which each use different amplifiers. My L/C/R channels are bi-amped, and the distortion is audible from both the HF and LF channels together.

Posted by Sam D. Chavez (Member # 1841) on 09-26-2016, 10:25 AM:

I would not discount the possibility of your setup oscillating. I had trouble with is unless there is at lest one connection from the low side of the Power Amp to the - or ground connection on the monitor. Once it starts ringing it will show up all over the place. Rebooting the 500 breaks the path but it comes back as you've stated.

Posted by Mark Gulbrandsen (Member # 72) on 09-26-2016, 02:42 PM:

I had odd motor boating issues with CP-500's & Crown amp's a long time ago, but that turned out to be improper wiring from the Strong factory where a ground loop had been built into the factory harness in each console...

Mark

Posted by Marco Giustini (Member # 4544) on 09-26-2016, 04:25 PM:

indeed, make sure it's definitely the processor and nothing else before giving up [Smile]

It shouldn't happen very often though. As I said it was a very rare occurrence - every several months approximately.

Posted by Tom Ostertag (Member # 8318) on 09-29-2016, 09:27 AM:

quote: Sam D. Chavez
I would not discount the possibility of your setup oscillating. I had trouble with is unless there is at lest one connection from the low side of the Power Amp to the - or ground connection on the monitor.

Very interesting issue! I don't actually have the booth monitor connected to the power amp outputs. This is my home setup, so I can easily walk out of the booth and listen to the speakers. I also wanted to maintain as much isolation as possible between the power amps and the monitoring, for reasons like you mention!

There are also distribution amplifiers between the processor and power amps, that also provide an isolated feed to monitoring. The booth monitor is not paralleled with the amps. I did that, in part, so that I could patch different sources directly into the booth monitor for troubleshooting purposes, without altering the signals or line termination to the amps in any way. These distribution amps have been thoroughly verified, but if the distortion happens again, I'll disconnect them and verify from the back of the processor itself.

The sound of the distortion sounds very much like it's happening in the digital domain before it hits the analog output chain. I've never heard this sort of "bit crusher" type effect as a result of analog circuitry or grounding issues. I won't rule anything out based on that assumption though. If (when) it happens again, hopefully it'll be when I have enough time to break things apart and test individually.