A review by Jeffrey Borinsky MIEE C. Eng.
Several years ago 405 enthusiasts had a choice between two high quality 625 to 405 line converters. I reviewed the Dinosaur and Pineapple units in 405 Alive when they first appeared. Unfortunately neither have been available for some time. There have been rumours of several new designs but nothing has appeared until now.
Domino is a compact unit which takes 625 video and audio inputs. The outputs are 405 line video and channel 1 VHF so no external modulator is needed. It is supplied fully built and tested. Power is supplied from an external adaptor, included with the unit.
Prospective purchasers may think that the price of £400 is rather high. The only reason why your TV sets and videos are cheap is because they are made by the million. When you are doing total production runs of 10 or even 100 the one-off costs of PCB manufacture loom large. And components are much more expensive in dozens than in thousands, let alone millions. Add in the hand assembly and test and I can assure you that the Domino converter is a bargain. Add in all the time spent in development it is clear that there is very little profit to be made and that it was largely a labour of love. The designer would probably do better financially by keeping his money in the bank.
Operation could not possibly be simpler. There are no external controls apart from a preset control for audio level. The construction quality is good. Inside the metal case are two PCBs, one for the main converter and a further one for the modulator. There are no user serviceable parts inside the case and I would not recommend users to open the case. Everything runs cool so reliability should be good. An external “wall wart” mains power unit, included with Domino, guarantees electrical safety and is an appropriate choice.
INTERPOLATION
Domino interpolates 2 lines from the same field. This gives the same
excellent interpolation quality as the Dinosaur converter which is difficult to
distinguish from the BBC CO6/509 which uses four lines.
SOUND
On the review sample there was no external adjustment for sound level. I
understand that all production units will have an externally accessible preset
control which allows the user to cope with the wide variety of outputs from
VCRs etc.
RADIO INTERFERENCE
The converter contains high speed digital circuitry which is a
potent source of RF interference. I cannot be sure that it complies fully with
the european standards but judging by the all-metal case it is unlikely to
cause any significant problems.
POOR QUALITY INPUTS
The framestore architecture ensures that the output syncs will
always be steady and continuous regardless of the input. In this respect it is
similar to the Pineapple converter and better than the Dinosaur. Very
occasionally you may see a whole frame dropped or repeated. This is an inherent
property of framestore based converters. In many years of working with
different framestore based systems I have never actually noticed this happening
and it is not a problem. The picture content may be garbage but the 405 syncs
will always be good. This is reassuring with some 405 line sets whose timebases
react badly to any sync problems.
While I cannot test the unit with all possible poor quality signals I can state that the input AGC copes well with low amplitude down to about –5dB. Note that the input video and sync amplitudes must be in the correct 7:3 ratio since the AGC measures the sync amplitude. Slightly noisy off air and ordinary VHS replay are fine too. It is always possible that really bad VHS replay could cause tearing or other effects but I have not seen this happen.
SOME MINOR PROBLEMS
DC offset
This is a very minor criticism and is of no real importance to users. In professional practice the black level of a video signal is at 0V with the sync tips at –300mV. This cannot be achieved without split +/- voltage supplies. The designer of Domino has decided that it is acceptable in consumer equipment for the black level to sit at +1V. This is unlikely to cause any trouble (I have seen worse elsewhere) but is something that could be improved. Please note that the absolute DC offset of the signal will not cause the displayed picture to have incorrect black level because the signal will always be AC coupled and DC restored or clamped in a monitor or modulator.
High frequency
response
This is the only aspect that caused me any real concern. Here is the engineering background. The 625 line system has a nominal bandwidth of 5.5MHz. Mr Nyquist’s famous sampling theory says that in a digital system you must sample at at least twice the highest analogue frequency. Hence 11MHz would be the absolute minimum and modern professional TV equipment uses 13.5MHz. You must prevent all inputs above half sampling frequency from reaching the analogue to digital converter. If you sample a 6MHz signal at 10MHz you will get a very nasty 1MHz pattern on your pictures reminiscent of 1.57MHz intercarrier/subcarrier beat. This effect is called aliasing and is prevented by a low pass filter at the input. Ideally this has no attentuation up to half sampling frequency and infinite attenuation above it. This is not actually possible and the practical filters used in professional equipment are very complex and expensive.
The Domino samples at 10MHz which made it easier to design but is definitely on the low side. Hence the theoretical maximum bandwidth is 5MHz with 4MHz as a practical maximum. Many inputs will be colour so the band around 4.43MHz must be well filtered to stop dot crawl patterns and aliasing. The unfortunate result of this is that the pictures are a bit soft. On test card C the two finest sets of HF gratings are not resolved. This will only be noticed on a really good receiver but is immediately apparent on the professional monitor that I used during this review.
Note that a 5.5MHz input at 625 will result in a 3.5MHz output at 405. This is due to the same number of cycles being strectched over a longer line.
I must re-iterate that although I am concerned about the HF response it is rare that you will have a vintage TV that can actually resolve these high frequencies anyway and the subjective effect is not disturbing.
The modulator uses crystal control to give 45MHz vision and 41.5MHz sound carriers. On the review sample these were accurate to better than 0.01% which is more than adequate. Sync level was correctly set just a few percent above zero carrier and the sound carrier was approximately 6dB below the vision carrier which is correct. There are no significant harmonics. Black level clamping was excellent with no noticeable shift or bounce as the picture was switched from full black screen to full white screen. There was no noticeable sound on vision or vision on sound. I am not able to measure the distortion in the sound modulator but looking at the design I have no doubt it will be much lower than in any likely receiver. A warning that the output is very powerful, useful for distribution or dealing with a very deaf set, but enough to overload many sets. An 18dB attentuator is supplied and should be used in most cases.
CONCLUSIONS
Despite some minor reservations I can strongly recommend the Domino as an invaluable product for all 405 line enthusiasts.
The Domino is only available directly from its designer:
Malcolm Everiss
26 Castleton Road
Swindon
SN5 5GD
01793 886062
email: malcolmeveriss@claramail.com
Price £400 including delivery. £10 discount if collected from a BVWS meeting. Cheque with order