Narrow-Bandwidth Television Association

Noise is something we normally avoid at all costs, in electronics as well as day-to-day life. But sometimes you need it to test the noise handling ability of some piece of equipment. Although I'm intending to use this for testing the NBTV/SSTV-625 up-converter it can be used in many applications by simply rearranging the output filter.

As shown the filter approximates a typical HF receiver's response of 300-3000Hz but the digital output has noise components up to 50kHz. It's based on a pseudo-random number generator which has 33 stages in the shift register, the random pattern goes though 2^33-1 patterns before repeating, about 8.6 billion. At the execution rate of the micro that's about once every 6.5 minutes - close enough to random for me!

You could do this in CMOS/TTL hardware but it would take six 14/16-pin chips plus an oscillator and the filter. I have seen a design for a hardware-based version that takes 913 YEARS to repeat using nine TTL chips plus oscillator and filter. It was used for gambling...which I abhor.

The maximum output voltage is +/-2.5V p-p. To measure the RMS value you need a true RMS reading measuring device with adequate bandwidth - not your average multimeter!

Steve A.

'The Brat', think of a noisy kid!

Actually I made a mistake, it repeats in 14.3 HOURS, not 6.5 minutes! I calculated using fewer stages than 33. Longer than your average working day of eight hours.

Steve A.

Steve Anderson wrote:Actually I made a mistake, it repeats in 14.3 HOURS, not 6.5 minutes! I calculated using fewer stages than 33. Longer than your average working day of eight hours.

Steve A.

Well i am glad i didn't calculate it could be in the years

I gather you are adding noise to your circuit Steve sorry but i am not sure where your connecting it ?

The noise generator is a piece of test equipment only used during development to assess how noise in the signal is handled by the up-converter. In a similar way it could be used for almost any communication channel. So it's not a permanent part of any device, it's used for a while and when you get (or don't) the results you want it goes back on the shelf.

The output will be mixed with the SSTV or NBTV signal in varying amounts and measurements made to find how much noise is too much and see if any changes made to the up-converter have improved things or not. In both these cases it's the sync detection that will prove the most important aspect to improve.

Steve A.

Thanks Steve i am a bit lost with the circuit and was wondering ...its a very neat solution to have it built in for testing.

Harry, as I said before, it'not built-in, it's a separate piece of test equipment which can be used in many different ways. It's like an audio bench-top oscillator which you can use on many different pieces of equipment, but instead of generating sine-waves it generates noise that is predicable in level.

Steve A.

Steve Anderson wrote:Harry, as I said before, it'not built-in, it's a separate piece of test equipment which can be used in many different ways. It's like an audio bench-top oscillator which you can use on many different pieces of equipment, but instead of generating sine-waves it generates noise that is predicable in level.

Steve A.

OH my mistake ,how do you adjust the noise level or is it the other way around adjusting the video level with it ?

What a brilliantly named device.

gary wrote:What a brilliantly named device.

Thanks Gary. Yep, something you generally don't want, but at least this one you can switch off.

Steve A.

Harry Dalek wrote:...how do you adjust the noise level or is it the other way around adjusting the video level with it ?

The way to use this is to have a standard 1V p-p signal (for NBTV) and gradually increase the noise. There'll come a point where noise will be visible in the picture, but more importantly, where the sync circuits start to fail. It's a bit subjective but you set a point in your mind where it becomes unusable.

Then you make some adjustments here and there and repeat. Is it better, the same or worse?

Most of us here don't transmit NBTV or SSTV but use a closed-circuit approach, flashdrives, e-mails, posting to this forum, none of which should introduce noise. Corruption, maybe, but that usually proves fatal to the data. Somewhere I have an article which claims that the circuits within are virtually immune to noise in a SSTV signal - I'm not convinced. But I'll breadboard it and see if the claim is true...somehow I doubt it.

Steve A.