Negating LF problems of NBTV.

Forum for discussion of electronic television. Generally, stuff to do with CRTs and not using mechanical displays.

Negating LF problems of NBTV.

Postby Steve Anderson » Thu Mar 26, 2020 6:27 pm

PC NBTV Interface 1B.gif
One of the biggest headaches of NBTV is the requirement of very good low frequency performance, Klaas Robers mentions on the NBTVA website, a figure of around 2Hz being ideal. Few audio devices have a frequency response down to such a low figure, even 20 Hz is often dubious. It's not so much the electronics but more often the record/playback media employed, whether it be tape, PC soundcards, whatever. Surely there's a way to negate this?

There is. While I'm stuck in the Maldives (I cannot get home due to no flights - COVID-19) I've come up with this idea...

USB ports can handle digital I/O at amazing speeds so why not make use of that? Combined with a Terminal Emulator program we have a fast (up to a point) serial digital I/O.

What this requires is a USB to TTL converter, I have access to two types. A USB to RS232 'dongle' is a possibility too...if fast enough.

Let's start with the software required on the PC. Forget Microsoft Hyperterminal, it ignores certain control codes and doesn't record them. I use Realterm, a freebie download with no ads etc. and goes up to 921,600 Baud which we need here. It takes a bit of getting used to, but it's worth it.

You'll need some form of USB-TTL conversion device, FTDI (UK) do 'cables' that convert USB into TTL serial data, just as if it had come via a RS232 to TTL device (a MAX232). Website here...https://www.ftdichip.com/Products/Cable ... Serial.htm Also distributed by the usual big companies RS/Farnell/Element 14 etc.. You'll be needing either TTL-232RG-VSW3V3-WE or TTL-232RG-VREG3V3-WE. Add your own connector to the bare wire ends, I used a 9D so I can use it on more than one external device. In this case all that is needed is TX, RX and GND. There are other sources too...Google it I guess...they're also available direct from the FTDI website via the previous link, as is the driver, on the same page (VCP drivers).

Add to that a DC-coupled video a-d/d-a conversion and an AC-coupled audio a-d/d-a conversion and you're in business!

So based on the above premise here's my plan...probably better explained in diagrams at this stage.

Click images for higher resolution version.

I'm not so keen on Butterworth filters for NBTV, they tend to introduce over/undershoots on sync pulses for example, but OK for audio, I may change them when I can experiment.
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Thu Mar 26, 2020 9:03 pm

Here's the byte format...

7 bits of video (good enough), and 12 bits of audio (also good enough)...

This comes out to 92,160 bytes/second, or about 5.5MB/minute. Not huge these days. About 3.5 minutes with the 20MB limit on this board. A lossless compression (Winzip) may help too.

The bit order may be reversed depending on software development - results in no practical difference.

Steve A.
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Thu Mar 26, 2020 9:33 pm

So, the workings out come to this...

921,600 Baud/10 (including start & stop bits (10)=92160 Bytes/sec. Divided by 3 equals 30,720 samples/sec, which equals 15.360kHz
of bandwidth for both channels.

I'm hoping at least adequate....

Steve A.
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Fri Mar 27, 2020 1:36 pm

For this we have a choice of basically two crystal frequencies, either 9.216MHz or 11.0592MHz as shown below. You may have to shop around to find these but I'm going to opt for the 11.0592MHz version.

My local supplier in Bangkok has over 25,000 in stock of the 11.0592MHz version so if you get stuck let me know...when I can get home that is...

Steve A.
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Fri Mar 27, 2020 3:11 pm

An alternative to the FTDI USB-TTL 'cables' is this small PCB. It does the same job using the same chip as the 'cables' and the same driver required (easy install). Also has all UART functions as can be seen on the right-hand edge connector. Selectable via jumper between 5V or 3.3V I/O levels. It's also powered from the USB port. The board is 56x43mm. The driver can be downloaded from the FTDI website in the UK, for free!

Only seen them in Bangkok as the company (ETT) are only 3km from my home. They only have a distributor in the US and Futurlec in Australia. Unfortunately Futurlec don't seem to carry this product, but many others. No UK distributor that I'm aware of. About half the price of the FTDI 'cables'at about GBP7.20 inc. local VAT.

The manual for this board is attached (in English). If you do visit their website (URL on PCB) be warned - it's all in Thai, no English. Exchange rate, 1GBP=40THB as near as dammit, don't forget to add Thai VAT at 7%.

Steve A.
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Fri Mar 27, 2020 10:08 pm

I've made a few changes to the above diagrams. The major one is the 3.3V power supplies have been raised to 5V as the A-D chip was marginal at the speed required on 3.3V. This also meant a change of processor to a PIC18F26K22, the previous one had a maximum voltage rating of 3.6V.

It's not worth re-posting the diagrams at this point as all other things are largely as before - including the micro's pin-out.

If planning to use the FTDI 'cables' that will need changing to a 5V version, TTL-232RG-VSW5V-WE.

The small ETT board will need the jumper set to 5V, as per photo above. It also requires a USB type 'A' cable as it's not supplied, the sort usually found on larger devices like printers. Unusually they haven't done a micro-USB version (yet).

Steve A.
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Sat Mar 28, 2020 8:40 pm

Quick update - I'm on a flight back home tonight!

Steve A.
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Re: Negating LF problems of NBTV.

Postby Dave Moll » Sun Mar 29, 2020 6:10 pm

That must be something of a relief.
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Re: Negating LF problems of NBTV.

Postby Harry Dalek » Sun Mar 29, 2020 6:46 pm

OMG i wish i could understand this ! .... not for lack of trying Steve just lack of brain power unfortunately... :oops:
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Negating LF problems of NBTV.

Postby Klaas Robers » Sun Mar 29, 2020 8:45 pm

Very good Steve! Be see you back from your own home.
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Tue Mar 31, 2020 5:39 pm

OK, back home now. Need some sleep, it was a long journey. I did some updates to the previous drawings whilst on the aircraft. More once I've had a snooze...

Steve A.
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Wed Apr 01, 2020 1:41 pm

Well, that 'snooze' turned out to be 16 hours!

Anyway, here's the UART speeds and bandwidth calculation results...

UART & Bandwidth Settings 1.gif
UART & Bandwidth Settings 1.gif (12.3 KiB) Viewed 551 times

Initially only the 921600 baud/15.36kHz bandwidth will be implemented in the code, it is mainly for NBTV use, the other rates/bandwidths may find applications elsewhere.

With the filters on the inputs and outputs the realistic bandwidth will be around 70% of that shown

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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Wed Apr 01, 2020 2:00 pm

Here's the updated main circuit..

Click for higher resolution version....right click....'Save file as..' follow the usual dialogue...once done, hit 'back to get back here...

I haven't updated the filters, I need to see how the original ones perform first. They also will need some gain/loss to get standard video and audio levels.

For (just) NBTV, 1A, 1B & 1C (IC101 pins 21/22/23) can be left open-circuit - though still with the three 100nF caps to 0v/GND.

Steve A.
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Wed Apr 01, 2020 2:10 pm

If you don't intend on programming the micro there's a bit of simplification that can be done...IC101 pins 27 and 28 are simply left open-circuit and JP101 omitted...
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Re: Negating LF problems of NBTV.

Postby Steve Anderson » Wed Apr 01, 2020 2:26 pm

The power supply is entirely conventional, and no heatsinks required...

Steve A.
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