What does a "Sound PM" include in the digital age?

[Dennis Benjamin]

I wanted to open a topic about sound. Specifically digital sound.

If a tech was to go out to a theatre location to conduct a "Sound PM" - what would it consist of? What tools would you need to bring and would things would you be listening for specifically?

Thanks.

[Tony Bandiera Jr]

It would be pretty much the same as it was for film, with the "A" chain now being playing test DCPs to check for proper channel assignments and audio quality.

The "B" chain side remains unchanged.

• Set EQ for each channel ( I always set rough levels, then EQ, then reference levels, then a quick recheck of EQ with minor adjustments as necessary.)
• Set proper reference levels for each channel
• Play test content, listen for failing drivers (rattles, crackles, distortion) or amplifiers/processors
• Make sure channels are sounding in the proper locations, correct as necessary and reset EQ and levels if amplifier inputs were changed.

Test equipment would remain the same pretty much as well. Mic multiplexer, RTA, pink noise generator (outboard or test materials), Known test content for reference (trailers, digital "Jiffy Test" type files) and a good set of ears and experience in judging sound quality.

Sound has been one of constants in the cinema world, with changes and upgrades along the way.

[Gordon McLeod]

What we do is
1 clean out all amps and anything with a fan
2 check tightness of all electrical connections
3 test each speaker that is working and doesn't rattle
4 Do a complete B chain and measure if there is issues with mechanical noise or bleed through between adjacent theatres (contractors are notorious in making holes in walls
5 Run a control piece of content that one is familiar with ( we made our own DCP we bring with us)
So for tools
We bring standard hand tools ( we use the Xcleite 99 serries as well as Wera tools
RTA ( we use the D2) with necessary pink noise and card extenders
sweep generator we use the Loftech)
SPL meter and calibrator IMG_20240119_221230601_HDR.jpg

[Harold Hallikainen]

An interesting question. Unlike analog equipment the digital equipment should not "drift." So, instead of drift, we should be looking for failures. SMPTE has RP 2096-2 Cinema Sound System Maintenance Calibration. Along with checking for broken stuff, this procedure includes a sine sweep on each channel while listening for rattles and other distortion. It includes a single microphone frequency response run that is compared with the same measurement in the last full calibration with the expectation that the response should be within 1 dB.

Another approach that I like (since I designed the equipment) is the use of the LSS-200 ( https://www.qsys.com/resource-files/...usermanual.pdf ). With it, a daily test show is run that causes the LSS to capture the white point chromaticity, and one or more SPL levels on each loudspeaker (use of band pass filtered pink noise in the test show allows the detection of failed drivers, crossovers, amplifiers, etc.).

Harold

[Marco Giustini]

I'd like to add a point here.

Sound is not like picture where you set colours and that's it, nobody can question that (though with Laser they could but that's another story).

The "X-curve" has a long story behind it. The bottom line is that it won't apply 100% to every single cinema out there. The acoustic of the room will change the way the "curve" looks when the room sounds "good".

So what I do when I do an EQ is to perform all the electrical tests, check phases, time align etc. Then I set the equalisers to reach the "X-Curve". After that, it's imperative to listen to known good content to evaluate. Particularly in small, dead rooms, the generic X-Curve might sound awful. Yes, there are many types of X-Curve (WinRTA gives you several options based on the size of the room) but it takes a little time and experience to experiment.

What I want to say here is that a properly-EQ'd room might not read a perfect "X-Curve". Particularly when someone checks with a single microphone with no spatial average.

I see around people "checking the EQ" by placing a microphone in the recommended position, changing the EQ claiming that it was off and then walking away without testing or maybe with just a few seconds listening test using a random trailer.

My personal opinion is that that is not ideal - and in fact you might have destroyed someone else's work who might have spent a few hours in that room tweaking and listening to get to a good result.

So, let's be careful with the "check the EQ" topic.

Now, that being said, we all know that the vast majority of cinemas around are in awful shape, missing HF, EQ all over the place. In that case, "checking the EQ" might be a good idea, even though it's done quickly.

Another point: an EQ or EQ check takes time.

[Harold Hallikainen]

Note that the SMPTE maintenance check with the single microphone only has you compare measurements with the results at that microphone position in the last full eq. If they depart by more than 1 dB, a full eq (multiple microphones, etc.) is called for. For those that worked in US AM broadcasting, this is similar to a "skeleton proof" where field strengths at various locations are compared with the field strength at those locations in the last "full proof."

[Mark Gulbrandsen]

Please note that humans can not distinguish a 1 dB difference. This is not only a well known fact, but was demonstrated at Skywalker sound during R2 training back in the early 1990's. No one in the group could detect a 1 dB change in either pink noise or steady state tone when listening in a dead quiet Foley Stage. But the majority there could detect a 2 dB change. Further, humidity changes can cause up to 1 dB up or down changes in sound systems. Just something to consider, and it's why they chose the 2 dB scale to do THX system alignments. 1 dB is fine for A chains or output levels of processors, etc.

As for dust in amplifiers, beware of just vacuuming off the rear panels, especially on QSC DCA amplifiers. Had a site where the amplifier would go off and then come back on all by itself. Wasn't an AC problem, it turned out that the amplifier was over heating because the sinks were clogged. If you encounter amps that have been in 5+ years, then you might need to pull them and open them up as dust can accumulate at the air input end of each heatsink... I've had plexs full of DCA amps out west that had the ends of the sinks completely clogged. If you have a bright LED flashlight, then you may be able to see in past the fan blades and look at them for inspection. Pretty sure I posted pictures of this on F-T in the past... Cleaning 6 or 8 screens worth of DCA's takes several nights!

[Timothy Eiler]

Until I retired at the end of the pandemic on a PM we did all that has already been listed above plus updating any software that was needed

[Pete Naples]

Run channel check DCP, Dolby Jiffy or similar, note any issues, back up setting BEFORE you start!

Software updates on the processor, crossover. Reboot first.
Clean processor fans, reseat/clean signal connectors.
Clean amplifiers, check connections. I often reseat and skoosh with Servisol/DeOxit. Especially in coastal locations.
QSC amplifiers seem to be notorious for failing mains switches and noisy gain pots. Check, order replacements/skoosh with DeOxit
Mains supply check, connections secure, earth good, voltage good.
Loudspeakers, check connections (JBL often have disintegrating plastic in the push-terminals), check mounting security visual check on condition. I tend to carefully move LF drivers by hand, listening/feeling for grating or sticking.
Run sweeps, listening for channels missing, farting/tearing/frying eggs etc. Basically anything that shouldn't be there, or isn't there.
Eq and SPL check
Surround delay check using test DCP
Lip-sync check
HI/VN check
Automation check
Back up settings from processor and crossover
Report.

[Mark Gulbrandsen]

This is why you check DCA's after they've been in a number of years...

[Marco Giustini]

Note that the SMPTE maintenance check with the single microphone only has you compare measurements with the results at that microphone position in the last full eq. If they depart by more than 1 dB, a full eq (multiple microphones, etc.) is called for. For those that worked in US AM broadcasting, this is similar to a "skeleton proof" where field strengths at various locations are compared with the field strength at those locations in the last "full proof."

That makes sense.

I've been in the industry for 25 years and I don't know anybody who would do that.

[Mark Gulbrandsen]

That makes sense.

I've been in the industry for 25 years and I don't know anybody who would do that.

Good luck getting the same tech out every time to have that happen properly. It's much more likely to happen that way in a commercial performing space.

Speaking of FIELD Strengths, when I worked in Broadcast, our transmitter was atop the John Hancock building in Chicago. Since we had an ERP of 1.2 million watts, we had to hire a service to run field strength for us, as we covered parts of four different States. Usually took them about 60 days to complete. This was done once a year, plus after we modded the transmitter to pulse operation to help cut the 28k dollar a month power bill down. And this was in the early 1980's. RCA TTU-110...

[Harold Hallikainen]

Interesting! The TTU-110 was a UHF television transmitter. I do not recall field strength measurements being required on FM or TV stations, only on directional AM stations. I spent a lot of time hiking through fields with a field strength meter measuring AM station field strength. This was before GPS, so I used a sighting compass and USGS maps to figure out where the measurement points were.

My only experience with Chicago broadcasting is installing a transmitter control system for WVON/WGCI in the late 1980s.

[Mark Gulbrandsen]

Interesting! The TTU-110 was a UHF television transmitter. I do not recall field strength measurements being required on FM or TV stations, only on directional AM stations. I spent a lot of time hiking through fields with a field strength meter measuring AM station field strength. This was before GPS, so I used a sighting compass and USGS maps to figure out where the measurement points were.

My only experience with Chicago broadcasting is installing a transmitter control system for WVON/WGCI in the late 1980s.

First and foremost, it was a Station requirement as far as knowing what markets we were covering so ads could be sold. They still did it yearly, and every year the coverage pattern did look slightly different than the last... likely just due to atmospheric changes. As far as does the FCC require it? I was told by the site engineer that only initially after the station signed on, so all knew it was meeting designed specs, and when any major changes were made to the transmitter like after we added the pulser to cut the power bill.. that actually worked very well by the way. Shaved about 10k a month off the power bill. Because the Hancock masts were frequently hit by lightning, and when it came back into the klystrons it would of course knock us off the air and also disentegrate a bit of the carbon internals in the visual klystrons. Enough hits and at least one klystron would be bad. So the offset of power cost helped pay for those 100k plus tubes...

I used to listen to WGCI in the past while driving into the city, seems to me they broadcast light rock music. All this was a loooong time ago, and none of the original Transmitters on those very top floors even exist any more. In fact even both building masts have even been changed to much shorter masts for DTV. I don't remember there being any radio stations up there...