Drive-In - recommended FM transmitter power?

[Sean McKinnon]

There are devices that will stream over wifi to smart phones/tablets/laptops etc... which can be blu-tooth enabled. If it were me I would probably look to use a DSP so that I can create my own "air chain" of limiting, compression, and combining.

[Carsten Kurz]

We had a few discussions here about using Wifi. With current technology and standards, the latency issues can not be solved for the requested amount of clients.

Operating an FM transmitter in germany is quite a bit more complicated than in the US. We need a license, and equipment is expensive and rare. Still, I don't see Wifi being an alternative. Patrons will not be amused with you dialing delays in and out trying to solve the puzzle.

- Carsten

[Marcel Birgelen]

Yes, this indeed has been discussed before on the old forum. Digital technologies relying on wifi and bluetooth (and even digital FM solutions) all depend on quite a lot of buffering. In case of wifi and bluetooth, the buffering takes place on many levels. So, the delay between the "stream-out" and effective playback is simply uncontrollable. If you pass-on your playback on your smartphone to the car audio via bluetooth, you're adding some other uncontrollable layers of buffering to the whole stack of buffers.

The puzzle might be unsolvable, because the delays may be different for patron A versus patron B, depending on what kind of phone and car-stereo he/she is using...

Unless you can come up with a nifty way to somehow time-code sync the playback on e.g. a smartphone with the stuff happening on-screen, those technologies are simply not going to work.

[David Buckley]

One of the other things I do is look after a community radio station, and we use the big versions of these transmitters; The V2 and V3 have adequate processors built in, with lots of different presets, but do spend a few moments setting the audio chain up like you would in a theatre house. Get your stereo L/R output from your projector, processor, whatever, feed it to the transmitter L/R in, preferably balanced, and put on a movie, something loud, and check that the loud bits are just bringing down the multi-band compressor levels, and then go and sit in a car and check it sounds OK, which it should. Too much compression and you'll sound like a terrible hits station, and it'll be really fatiguing to listen to, you just want to trim the top. Once it sounds good - leave it alone!

[Harold Hallikainen]

It seems like there should also be some HF rolloff added (about 7 dB at 10 kHz) since you are not shooting sound through a screen.

[David Buckley]

Unfortunately, the BWs don't have EQ in them, so such EQ would have to be placed externally, previously to the transmitter. The processor in the transmitter, useful as it is, is a preset-only processor,.

[Barry Floyd]

I need to take some time to research compression. Initially our sound was great after installing digital, I found later last year some of the louder scenes started to sound "muddy" to me, though still fine during normal scenes. I'm always very nervous tinkering with the mixer during the show, even though I can see the "clip" light flickering and the TX5 reading " ! ". Not sure whether the DCP's are outputting higher levels lately, but wasn't an issue before.

We were having the same issue. We ended up installing a Compressor / Limiter between the USL JSD-60 and the TX-5 transmitter. Solved all of those clipping issues.

[Juan Jose Garcia Calvo]

We were having the same issue. We ended up installing a Compressor / Limiter between the USL JSD-60 and the TX-5 transmitter. Solved all of those clipping issues.

Can you share with us the compressor / limiter settings (ratio, release time, ....) and if it's a multiband compressor, what settings could we use for each band, and what split frequencies...
This culd be very helpfull

Thanks

[Carsten Kurz]

We did our first (small) drive-in show yesterday. We used a small and low cost digital mixer, which offers a compressor on the main busses. We simply optimized it by ear. There are expensive devices optimized for commercial radio broadcast, but I think we don't need that for cinema, as in a drive-in, we don't take part in the common loudness war.

I was very satisfied with the results of the simple X-Air mixer, also because we can operate it remotely through Wifi. Which is quite practical, because you can have your gear 'somewhere' while you sit in your car with an iPad and tune things.

Many car audio systems nowadays are quite capable, and I think it is not necessary to apply too much compression.

I was trying to find typical suggestions for optimizing EQ and compression settings on the internet, but I didn't find much, most hits dealt with commercial software and processors. I guess if you look a bit longer, you will find some, but I decided to simply hook up the gear and try myself.

[Marcel Birgelen]

The compression is partly needed to keep within legal requirements regarding your allocated FM band. We're lucky to have a "cheap" Orban processor at our disposal and a broadcast-tech is going to tune it for us. I told him that I don't want dialog to sound like the average outraged radio presenter, so we'll see where we end up.

What movie did you play? And did you play it from a DCP source?

[Carsten Kurz]

We played a short-film program. For operational reasons, I converted the original Bluray to DCP. It was a small scale operation, but it allowed us to do some real life tests for larger scale operations.

[Marcel Birgelen]

Did you have a possibility to drive around a bit to see how far your little "pirate radio" is getting?

I've been toying around with our little setup, we're still waiting for the green light from the local authorities, as right now it's not clear if a drive-in cinema violates current rules against mass-gatherings... apparently, the authorities also don't really know how to interpret the ad-hoc rules in place.

At first I was thinking about using the CP750 I recently repaired, but instead we're using a digital mixing panel (with EQ options) with AES inputs to do the down mixing from 5.1/7.1 to 2.0 as this should give us maximum flexibility. I was wondering how everybody is summing up the channels. Right now, I've done a 1:1:1 on all channels to the left and a 1:1:1 on all channels to the right. The LFE also gets dumped 1:1 to both right and left. The sound didn't yet go through the compressor, as someone else with some real Orban knowledge will be setting that thing up.

I used this as a starting point and I was only listening through a bunch of headphones, but it's clear that simply dumping the individual channels "one on one" isn't really the right thing to do, especially in high-octane action scenes with a lot of surround action going on, it's clear the surround channels are way to present. I started to manually tune the surrounds down and the LFE a bit up to a level that it was more comfortable. I also toyed a bit with the EQ, but all in all it sounded pretty decent, almost flat.

So, I was wondering if anybody else is "manually" downmixing their channels to 2.0 and if so, to what proportions do you mix in the surrounds and the LFE?

The biggest uncertainty for me is the LFE. Should I boost it with 10dB or is that just too much for FM transmission?

[Carsten Kurz]

That's an interesting discussion. As a lead in: Occasionally I service a small cinema, more a 'film club', quite a distance from here (NC900). I did supply some surrounds at some point, but they disconnected them later because of complaints. The room is just to small to give an ambient surround experience. It was simply too directional and distracting for 'their' audience. Well, some time later, they played 'Roma, and somehow someone noticed that there was dialog missing. Turned out, the Roma sound mix was sophisticated enough to have a lot of dialog in the surrounds only. That made them rethink their 'position' with regard to surround speakers...

Whatever:
We do all the mix down on a per channel basis in a conventional PA style mixer. So far, for our OpenAir screenings we used a small analog PA mixer with 4 outputs (two main, two groups), so we could have L/R/C and mono surround mixdown from an analog 5/5.1 input.

Now we use a digital mixer. It's a pity it doesn't offer AES inputs right away, but at the price point, it is simply not possible. So we use an otherwise unused DMA8+ in the mobile equipment rack. Doremi/Dolby AUD2DA or USL DACs would be other options.

We feed L/R/C/Ls/Rs into the mixer and leave away LFE. We use three screen speakers with 15" woofers, so I think it's okay to skip the LFE. We usually have between 300-500 people at our openair showings, and the listening situation is near optimal (a 'raked' seating area facing the screen, almost like a greek amphitheatre ). For a larger crowd on a broader estate, we would probably need to add separate subwoofers.

For FM transmission, LFE should be completely unnecessary. I understand, though, there is usually some upper spectrum in the LFE that could be reproduced by car audio systems, so, if you are keen to play and tune, one could probably use a HPF on the LFE and add it to the mix, so those who spent shitloads of money on their car audio systems and have no space left in their subwoofer-cabinet trunks will have fun.

I don't know what an Optimod would do with an LFE signal if it sees one. BTW - what minimal overall latency is possible with an Optimod? Because DCI servers will only allow 280-320ms of timeshift between audio and video, and, for classic radio broadcast, they usually don't care about such small latencies. I know some smaller stations use PC/software based signal optimizers/MPX encoders with in/out latencies of several seconds.

If you use a bluray player, even less delay may be possible in it's AV-Sync setup. To the point where you may need to master your own DCP with a longer pre-delay.

As far as surrounds go - I guess here is why a conventional digital mixer could be a very nice solution because of it's flexibility - you can use separate per channel compression/limiting/EQ, so, instead of just reducing surround levels, you could use compression and limiting, so you can make sure, surrounds are there, but not take over.

For the short film program, we only used a classic compressor. I will take the time now to tune a 5ch multiband compressor and the limiter in order to keep signals in compliance with local MPX constraints. This is, of course, not a commercial broadcast business, so, I am not trying to be as ambitious as they are with maximizing spectrum usage. Keep in mind - FM transmission is targeted at household and in-car receiption, for non-attention-paying audience, and with a lot of environmental noise, especially in a driving car. We can utilize quite a bit more dynamic range in a drive-in cinema, especially since currently, windows have to stay closed.

That small Behringer X-Air offers a heck of signal processing and routing options, and as I said, it is extremely practical you can sit in a car and tune your signal using your iPad while listening to your own standard car audio system (I consider mine as fairly outdated and average, so, I guess not a bad starting point).
The X-Air also offers nice addons - like an integrated USB-Stick based audio player - you can have it playing looped announcements/hints during 'admission', so people have something to tune their radios to when they arrive. Multiple Mic inputs for live announcements, instructions and warnings.

And, of course, everything you do can be saved and recalled easily. It is saved to the unit, and can be called up even if you connect with a different iPad or notebook. Heck, it's 'almost' a Q-SYS ;-)

[Marcel Birgelen]

I was just listening with a pair of Sennheiser studio headphones and I was somehow missing the LFE withouth the +10dB boost. Obviously, the headphones will not be able to render the lower band of the LFE, but there's indeed still quite some useful information in the upper bands as well. Putting some kind of HPF on it might be a good idea, if the Optimod doesn't take care of that already. Otherwise, the EQ on the mixing panel can get the job done. Most sophisticated car audio systems do bass management anyway and many of them boost the bass, so it's probably best not to overdo it and put it somewhere in between +3~6dB range. As for the surrounds, I think I'll keep them at around -3dB, and give the center channel a little kick, but it might depend a bit on the audio mix. The problem is that the surround channels seem to mud out the dialog from the center channel otherwise.

I don't know much about the Optimod yet, I'm a bit intrigued by the whole thing, but I'll start to play around with the next few days. Luckily, we've got someone who actually configures those things for a living. I guess it will probably filter the lower frequencies from the signal, but I'm sure there will be some LFE information left in there. The thing also features some special filters that Orban is known for and which is the secret sauce to the "presence" of many known radio voices. Also, stuff like de-essing will probably not be needed. And yeah, I guess we don't really need that secret sauce. What I'm more interested in is the apparent "world class" automatic audio-normalization/compression features the thingy has and how it will affect the audio dynamics of a modern movie. As for the latency, I was assured it was "just miliseconds", because it's a dedicated hardware solution and all processing takes place in dedicated DSPs, so for now I'm not so worried about my ~300ms budget I have on the playout side of things.

I'm going to take a look at the Behringer X-Air too, which might be a good but affordable solution when we don't have some people who do radio broadcasting for a living as a support team.

[Carsten Kurz]

Yes, that's the problem, L/R/Ls/Rs/LFE will smother Center in a summed mixdown compressing scheme. I think in the X-Air, one could setup a combination of compressors and limiters on a channel basis that would take care of that.
E.g. have a ducker/sidechained compressor on L/R/Ls/Rs/LFE fed by the center channel. That would always keep the center channel on top IF there is signal on it. Sounds like a lot of fun trying...