DMX frame rate question and suggestion

[Arjan]

Hi,

Could you possibly tell me the actual DMX frame rate that can be achieved with ShowXpress in combination with a D512 interface using the High setting for the DMX Streaming Frequency? Is this a fixed rate or does it somehow depend on the speed (and type?) of the host computer? Is there any way to get a readout of the actual DMX frame rate from the software somehow?

Also, would it be possible for you to increase the maximum frame rate by allowing the user to specify a reduced number of DMX channels to be transmitted? For example by limiting the DMX channel count to 256 would it be possible to achieve double the normal maximum frame rate? And similarly by limiting it to 128 channels could you quadruple the frame rate? This would be totally within DMX specs and is considered an acceptable to achieve higher frame rates as pointed out here.

I'm guessing that I'm not the only one who doesn't use or need a universe with full 512 channel capability so if reducing the channel count would somehow open the door to higher DMX frame rates please consider this a feature request.

Specifically I think a higher frame rate might result in smoother fades in LED pars as even with 44 updates per second for a full 512 channel universe the value would have to increase by 255/44= 5.8 on average per update for a fade from 0 to 255 in one second. Although the human eye can be fooled into seeing smooth motion with frame rates >18 it is capable of detecting changes in intensity at much higher frequencies and I think that's what I'm seeing when I do a fade as described.

Thanks,
Arjan

P.S. I know that some LED PARs themselves may also contribute to this phenomenon but I have the impression that with my Puck Q6/W LED pars most of this effect is coming from the limited DMX frame rate with which they are being controlled.

[support]

Effectively, many "Chinese" leds lights do not "integrate" the dimming curve. The result gives, in certain conditions, "steps" during dimming. This is a pity because it is easy and cheap to integrate this electronic circuitry into each light. The result is users push us to increase dmx refresh frequency.

Unfortunately, we can not increase the dmx speed refresh. Only "pure electronic" lighting desks (no computer), or USD 50K (and more) lighting desks can reach high dmx refresh frequencies, but not a Windows based computer.

[Arjan]

Effectively, many "Chinese" leds lights do not "integrate" the dimming curve. The result gives, in certain conditions, "steps" during dimming. This is a pity because it is easy and cheap to integrate this electronic circuitry into each light. The result is users push us to increase dmx refresh frequency.

I understand that internal interpolation in LED fixtures would be very desirable. In fact I had already sent an email to the manufacturer of my lights (Puck Q6/W from Blizzard Lighting) asking if this is something they do and if not if they could include it in future versions of their product.

Actually smooth dimming is not the only reason I'm asking about the DMX frame rate. I find that I'm having some trouble getting repeated short flashes (single strobes) quickly after each other. For example I want to tie a button for a flash to a drum trigger that is mounted on the snare. I'm still researching this but I do get the impression that even when I'm directly sending pre-programmed MIDI messages to ShowXpress (so not involving the drum trigger module) that sometimes ShowXpress misses some of these triggers, ie. the second flash won't show.

Again I'll need to look into it further to make sure that it's not my own software that is partly causing this but it also seems to happen if I click the button with the mouse (but perhaps the second click only comes through as a WM_LBUTTONDBLCLK rather than a second WM_LBUTTONDOWN or something). I'm somewhat guessing that the D512 is running the DMX stream asynchronously from the updates it receives through USB and that PC/software is the limiting factor here, correct?

The problem with the flashes may also have to do with the MIDI messages that I'm sending or the sequence I've created that responds to them. Do you have a recommend way of programming such 'flash' sequences and how to control these by MIDI for maximum performance? Might I get better results if I duplicate the 'flash' sequence and alternate between triggered the original and the copy?

Unfortunately, we can not increase the dmx speed refresh.

Somehow I would think that if you can do, say, 20 updates per second with 512 channels it should be possible to reach somewhere close to 40 updates per second with only 256 channels or even just 128 channels. I can't imagine that the number of channels being transmitted is not a contributing limiting factor in terms frame rate.

[kahilzinger]

Actually, it is. DMX sends all 512 channels whether you use them or not. There is no compression and since there is no transmit from the fixtures, there is no error correction.

[Arjan]

DMX sends all 512 channels whether you use them or not.

Not true: http://www.plsn.com/index2.php?option=c ... =1&id=3729

NEW YORK—A new edition of a lighting protocol that started with DMX 512 is now available. ANSI E1.11 - 2008, Entertainment Technology - USITT DMX512-A, Asynchronous Serial Digital Data Transmission Standard for Controlling Lighting Equipment and Accessories, can now be purchased from The ESTA Foundation. The path to the document starts at ESTA Foundation - Publications - About Publications, Browse & Purchase . The list price is $40; member and quantity discounts are available. ANSI E1.11 - 2008 is the most recent updating of the ubiquitous lighting control protocol that started with the USITT DMX512 standard in 1986. It corrects errors and clarifies text in the 2004 edition and adds functionality.

An important clarification is the maximum update rate. The maximum rate is 44 Hz if 512 data slots are being sent, but it can be almost 20 times faster if a data packet contains fewer slots. This has been true ever since the original USITT DMX512, but some receiver manufacturers have missed or ignored this detail.

If you send the minimum number of slots (channels) per packet (frame) it's possible to achieve a 829Hz frame rate. Not all fixtures may be able to cope with that though given the emphasized bit from the quote (emphasis by me), but it is and always has been completely allowed by the DMX standard.

This information is also confirmed by several other sources. Not many controllers may allow reduction of number of channels sent but some do and fixtures should be able to deal with it if they want to claim 100% conformance to the standard. I'm sure though that quite a few will not handle 829Hz well or at all.

Having said all that, I'm not even sure that the current version of ShowXpress is outputting the standard full 512 channel 44Hz frame rate, it appears to do less updates than that. Too bad because it would make a very nice piece of software even better. I'm convinced that technically it's possible to get these and even higher frame rates from a standard Windows PC under normal no-stress conditions (if Windows starts page swapping naturally all bets are off).

[kahilzinger]

I would defer to support, but I don't think they allow that. At least not without firmware, EPROM or hardware changes.