New Aurora converter - help wanted with review
 

I have the new low cost Aurora standards converter on the bench for review at the moment and apart from a couple of minor problems it's working well.

I could do with some help on the RF side of the review. The only 13 channel TV I have working is not a very good set and far from ideal for assessing the multichannel modulator. Is there anyone not too far from me with a good working 13 channel TV (or more than one of them!) who could help. I'm happy to travel to you within a reasonable radius of North Finchley or you could visit my lab.

Your help will receive due credit in the review that I'm writing for the BVWS Bulletin.

Re: New Aurora converter - help wanted with review
 

Hi Jeffery,

I'm sorry but i can't help you with the 13 channel tv, but i would be very interested in your review of the converter.
I have just e-mailed Aurora to find out if these are now generally up for sale as i am seriously considering buying one.
I would much appreciate anything you have to say about this piece of equipment.

Cheers (and good luck finding the tv for the review),
Alan. :thumbsup:

Re: New Aurora converter - help wanted with review
 

If you fancy a trip up to Birmingham I have a Pye 17" 13 channel setthats in good working order. And several 625 sources to select from, Computer, Video, DVD etc...

Andi

Re: New Aurora converter - help wanted with review
 

Thanks Andi but I was hoping for somebody a bit closer. Otherwise I may post it up to David Boynes who has a lot of good sets and experience of evaluating converter performance.

Re: New Aurora converter - help wanted with review
 

Hi,
interestig are the non superhet sets (is this called TRF?) like LV20 TS1105 because they catch
noise near the channel too.
Kind regards
Darius

Re: New Aurora converter - help wanted with review
 

Hi Jeffrey,

What about my sony 9-90?

Maybe a screen shot of a Spectrum analyser showing the RF output?

Cheers
Sean

Cheers
Sean

Re: New Aurora converter - help wanted with review
 

Sean, that's effectively what I'm using except it's the monitor version CVM-90UB. It's hardly a set I'd use for critical assessment. I've already looked on the analyser and seen loads of harmonics which don't matter in a closed circuit environment. Otherwise the RF output looks fine on the analyser. What I need to look for is more subtle things like patterning and sound/vision crosstalk. I've got a suspected patterning problem on channel 10 which could easily be the set and a possible digital breakthrough problem on channel 3.

I'm trying to get hold of John Thompson (not a forum member) who's not too far from me and has a selection of sets. I may also go down to Gerry's museum though he doesn't really have 13 channel sets on display.

The review is going well otherwise. The video output is excellent (no surprises there) and a couple of minor problems I've found will be ironed out before any are delivered to customers. I just need to get a good look at the modulator performance and then I can submit the review to Carl for publication.

Re: New Aurora converter - help wanted with review
 

Hi Jeffrey, the problem is to suppress noise in the channel and a few MHz
up and down. If you have a 50% white screen there should be no moire
visible. The suppression of noise in the channel should be much better than
60dB against peak white.
To put the modulator and converter in one box is not easy!
An unfair trick is to blow the modulator level to more than say 95dB and put an attenuator in front of the set. I am not sure about the motorola IC stand
alone on the low VHF frequencys. The results of the axing there don't make
me optimistic.:-/ But axing is not aurora:)

Kind regards
Darius

Re: New Aurora converter - help wanted with review
 

Hi Jeffrey,

I have just purchased a large Ekco (14"?) set, I hope to collect it this evening, and maybe get it working on Saturday - I wonder if this would help you with the review?

Cheers
Sean

Re: New Aurora converter - help wanted with review
 

Thanks to all of you who have offered help. I went to Gerry's last night and will be visiting a local forum member tomorrow. I hope this will allow me to complete the review.

Re: New Aurora converter - help wanted with review
 

My review will appear in the next BVWS bulletin. Thanks to Gerry Wells, John Thompson and Graham Davis who made TVs available and gave encouragement.

I don't want to pre-empt the printed review but there is no doubt that the new Aurora is a very good design and good value at around £150. At the NVCF we had the chance to compare the Aurora directly with David Robinson's design. Both modulators use the same Freescale chips. On ch9 the performance was almost identical, even when we attenuated the signals which tends to cause more patterning. We did not have time to do comprehensive trials on all channels but we noted that ch10 was still the most susceptible to patterning though the effect was small. To minimise or eliminate patterning there are 2 golden rules.

• Don't attenuate or split the signal (use a distribution amp to feed multiple sets)
• Use good quality cable and connections.

Re: New Aurora converter - review now published
 

My review of the Aurora converter has now been published in the BVWS Bulletin. I am now happy to publish the text here.

Re: New Aurora converter - review now published
 

Yes, I noted that it is in print.

I'm looking forward to reading it at lunchtime tomorrow. It probably means I'm going to get hooked on the vintage TVs as well as wireless - thanks! :-)

Regards,

Re: New Aurora converter - help wanted with review
 

Good morning ,
thanks to Jeffrey for this very interesting review.

It made me thinking and I have some technical questions.

On Page 15 BVWS Journal DC offset you wrote:
„In this design the sync tips are at about 0.2V which will not cause any practical problems.“
Page 33 „...the simple emitter follower output stage.“
So the sync tips are not on the zero rail?! How is this stage made? How much ohms is the pull down resistor of this emitterfollower ?

Another question in the modulator sync tip level page 15.
„The Aurora’s modulator puts them at 10%.“ Means modulation depth is 90% in your case.
In the aurora manual page 20 is written „99% maximum“
But have a look at the data sheet attached:
minimum 87% typical 93% maximum 99% What value should we take?
And with the sound ist is the same, because it goes via the vision input of the other IC.

Kind regards
Darius

Re: New Aurora converter - help wanted with review
 

I think that Darryl should answer these questions but here is my opinion.

The output stage is a simple NPN emitter follower. The emitter resistor is 200R. The base is biased slightly positive.

I measured the minimum vision modulation as 10% at sync tip. I did not look at the manual or the data sheet. It does not matter if the sync tip is anywhere between 0% and 10% provided it is reasonably stable.

I did not measure the audio modulator in detail. If it can only reach 90% modulation I would not regard this as a problem. This is equivalent to 1dB down from the maximum possible. A 1dB difference is just audible by direct comparison

Re: New Aurora converter - help wanted with review
 

Yes, I agree. And maybe you'll get a 99%.;D

The emitter follower with 0.2V sync tip and 200 Ohms pull down causes problems with some for example vintage DIN AV inputs on VCRs. They have a cap in front of the 75 Ohms load and this will cut the sync signal.:( I've seen some modulator schematics using cap loaded emitter inputs too. So this vision output can cause trouble.

Kind regards,
Darius

Re: New Aurora converter - help wanted with review
 

If there is a cap before the 75R termination that is bad design.

Re: New Aurora converter - help wanted with review
 

Yes, definately. But they exist. Was it a common french modulator we discussed here?
It is not a problem to drive them, if you change the biasing of the emitter follower. For example 1 volt at the sync tips. In this case there is enough pull down current.;)

Kind regards
Darius

Re: New Aurora converter - help wanted with review
 

If the modulator is like my design, the sound modulator is set to System I, negative vision mod, and the input is biased by a preset pot which acts as an adjustment for the sound/vision carrier ratio. With this arrangement, 100% sound modulation depth is not a problem, in fact it will go beyond 100% (inverted carrier)! This is because the peak sound power on 405 should be equal to the peak white vision power, but the modulator when working with negative mod, has more than peak white to sync tip range (as peak white is 20% carrier).

Re: New Aurora converter - help wanted with review
 

Hi David,
Sorry,I don't agree. I think there is a confusion with modulation depth and sound/ vision carrier ratio. These are different and independent things.
If you use the vision input for sound and switch the IC to negative vision mode (means PAL vision mode SYSL=0) you'll get something between 76% and 88% typical 82% modulation depth.
If you use your bias pot for the sound/vision ratio adjust it will be lower.
There is a biasing for maximum modulation depth, the sound/vision ratio adjust must be made with the output adding resistors.

Kind regards,
Darius

Re: New Aurora converter - help wanted with review
 

Hello all,
Sorry for the late entry into the thread, but I just got my third kidney stone :-O

Anyway both Jeff and David have correctly spoken. First, the composite output from the converter is a simple NPN emitter follower. Since the main goal of this design was to offer the best conversion quality with a built in modulator, very little cost was allocated for the composite output which would probably never be used. Since the RF modulator is built in, there is little need for the composite output. It was added at the last minute since there was physical room in the box for the connector. Also, as Jeff has pointed out, when driving a correctly terminated 75 ohm load, the output performs properly. It's main benefit is that it offers the full video output bandwidth as the sound trap filter is after this circuitry.

For the modulation specs, the depths quoted in the data sheet are for a 1Vp-p input, typically from a video source, and account for variations in the modulator IC's themselves. As David has stated, these modulators can be driven to full 100% and beyond which will eventually result in a reversal. I do operate the AM audio modulator in the negative video mode as David mentioned, and supply a biasing pot in the audio op-amp circuitry to set the nominal level.

Hope this helps.
Darryl

Re: New Aurora converter - help wanted with review
 

Thanks for answering here and best wishes to you, Darryl.

Ah, you are right.:D

I did not know that the peak white clip (PWC=1) can be switched off in the negativ vision mode (SYSL=0). This makes it possible. So the values in Table 21 "PAL video modulation depth" are not guilty for your sound application.

Well done!

Kind regards
Darius

Re: New Aurora converter - help wanted with review
 

Good morning,
the vision output will be used to record something on for example vhs tapes or to drive another modulator. Often old vcrs are used and they sometimes have a cap and resistor terminated input. This design is used to make it possible to use the same pin for vision in and output (DIN-AV). You only have to change the biasing to make it possible to drive these inputs. No cost and no room needed.;)

Kind regards
Darius

Re: New Aurora converter - help wanted with review
 

Hello,
I have just purchased an AURORA converter and all I can say is that it produces a VERY good picture reproducing the 3mc/s bars easily on my 14" GEC BT1748 1956 receiver. It will match the quality of the original BBC signal and its about as good as you can get. I'm happy! Regards John.

Re: New Aurora converter - help wanted with review
 

Anyone able to show us a screenshot from the spectrum analyzer? (modulator set to London ch1)

Darius

Re: New Aurora converter - help wanted with review
 

Hi Darius,
While this will potentially help, it does pose some engineering challanges. Increasing the load on the transistor will increase the base current which needs to be accounted for in the D/A section. Also, this will move the hfe curve further into a non-linear region which will affect the output. I've attached the schematic of this section to illustrate.


I can do this Darius. What specifically are you looking for? (Span, RBW, video content?)

Take care,
Darryl

Re: New Aurora converter - help wanted with review
 

Oops, forgot the attachment.

Darryl

Re: New Aurora converter - help wanted with review
 

Re: New Aurora converter - help wanted with review
 

Good morning Darryl, thanks for the schematic. I am impressed what can be done in one IC.8-) My analogue design is much more complicated:( but discrete:).
You are using an R-2R D/A section. The load is not increased by changing the biasing of Q1. The base current is not importand for the R-2R and the filter.
The input resistance of the emitter follower does not change with the biasing because R36/7/8 are still the same!
It is 40K Ohms without the biasing resistors R34/35.
Vice versa you are getting into a more linear region with a higher biasing voltage because the ratio of AC current changes to DC current of the transistor gets lower by higher biasing.:) With your biasing the non linearity is most at the sync (tips). They are square wave and the sync will be made a bit shorter at the output. You can take every value for R34/35 you want, if you make sure that R34 II R35 does not change.

Attached some screenshots from the analyzer with my modulator without filters at the output. Input is a colour bar without colour.

Kind regards
Darius

Re: New Aurora converter - help wanted with review
 

I hope this is correct. The output impedance of a R-2R ladder DAC is R (in this case 1K) regardless of the digital code. I don't think the departaure from the pure R-2R structure affects this. There are very small variations with code due to the finite output impedance of the digital drives to the DAC. These are not important.

Hence the Thevenin model of the DAC is a variable voltage source (0-3.3V) in series with a 1K resistor. The resistive load on the DAC affects only the gain, not the linearity.

The base current of Q1 is very close to Ie/HFE (large signal parameters). This will vary between devices and with temperature. The input impedance of Q1 is Re/hfe (total load on emitter divided by small signal current gain). The signal is actually large in this application so hfe will vary with the signal and cause non-linearity.

I think that the resistance of R34//R35 will dominate the load at the DAC output so the hfe variations will not cause any significant non-linearity.

Re: New Aurora converter - help wanted with review
 

Good morning Jeffrey,
I totally agree.
R34//R35 (// means parallel) sets the output level.
A higher biasing makes the non linearity "unmeasureable".

Kind regards
Darius

BTW The output level at VID_CONV changes a little bit with the load.
It is visible! It is better to take a separate emitter follower for each output.
(It is the same with opamps!)

Re: New Aurora converter - help wanted with review
 

You are correct. The output amplitude will change with loading due to finite output impedance. Separate emitter followers are better but in this case Darryl took the decision to use a single emitter follower.

It is NOT the same with opamps. The closed loop output impedance of an opamp is very low, much lower than an emitter follower. In video applications I use opamps to drive multiple 75R loads, each output via its own 75R resistor. Some opamps are better than others for driving several loads. The EL2x44 series will drive 2 loads, the older EL2020 will drive 3, the LM6172 will drive at least 4. The interaction between outputs is almost unmeasurable (<<0.1dB) at low frequencies and may increase up to 1dB at 10MHz depending on the opamp. For the highest quality applications the maximum loading is usually set by the rise in differential gain and differential phase.

Re: New Aurora converter - help wanted with review
 

Hi Jeffrey, I know this from the commercially available video splitters.
You always "see" something if something is done on an other output.
(With difficult loads, some start to oscillate:( This is the kind with 1Vpp and 0R or a coil (?) at the output:'(.)
Good splitters have an opamp for each output.:)

Another Question:
I had a look at the schematic.
There are seven bits generating the vision signal. Means 128 steps.
Sync takes 30% means there are only 90 grey steps left for the vision.
Am I right?

Kind regards
Darius

Re: New Aurora converter - help wanted with review
 

When correctly designed you need only 1 opamp. I have designed very high quality equipment with multiple video outputs to be used by broadcasters. I have never used one opamp per output. The isolation between outputs is always good. You can even put a long unterminated length of cable or a short circuit on 1 output and the effect on the others will be very small. The 75R resistor in series with each each output ensure that the load on the opamp is always mainly resistive.

No. This is a 10 bit DAC! VDO9-VDO6 are the 4 MSBs as in a normal R-2R DAC. VDO4/5, VDO2/3 and VDO0/1 are pulse width modulated signals and represent 2 bits each. The output is sampled at 3 times the normal speed. Here are the pulse width modulated waveforms:

00 ____________

01 ____--____--

10 __----__----

11 ------------

These waveforms are integrated by the filter to give the correct average values. The resistor values are different for these lower bits. You have to sit down and do a lot of calculation (mainly using Thevenin's theorem) to prove how this works.

I have designed pulse width modulated DACs but only for very low speed control systems. In one design I use a 12 bit PWM DAC that requires only 1 pin on an FPGA to adjust the fine tuning on a TCXO. Darryl's design is a hybrid between R-2R and PWM and is very clever. It is probably more accurate than using extra resistors. The main reason he did this design was shortage of pins on the FPGA.

Re: New Aurora converter - help wanted with review
 

Hi Jeffrey,
thanks for explaining this. It is new for me.:) So there are 700 grey steps.
This is good!:)
To compare it with an analogue world: The CXL5506 has typical 56dB s/n
things are visible more than 6db below noise makes 56dB+ 6dB say 60dB
this is 1000 minus 30% (sync) gives 700, too.

BTW: 0,1dB is 1% this is definately visible!

Kind regards
Darius

Re: New Aurora converter - help wanted with review
 

Hello Darius and Jeff,
All the discussion about the output driver seems spot on. There are non-linear effects caused by the follower, but regardless of the load, they are minimal. The output impedance of the D/A is 1K as Jeff pointed out, regardless of the digital code or of the modified nature of the pwm bits. R34, the 2.32K, is installed (R35 is not) so the effective D/A output impedance is about 700R. Both R34 and R35 are never installed at the same time. Only one is used to set the D/A gain and operating point.

The hfe curve for the BC848 is the flatest between approximately 1-30ma which is where it is biased to operate currently. Beyond 30ma the hfe has a pronounced rolloff that would result in a change in gain for the video portion of the signal. I believe Darius correctly pointed out that the sync portion of the signal is most affected by the low end of the hfe curve now, but pushing the biasing beyond 30ma would mean the video portion would be affected. BTW, the sync signals generated are not just square waves but are edge rate controled as required.

Darius is correct that independent followers would be ideal, however this isn't required. While there will be a gain change to the signal fed to the internal modulator (VID_CONV) when an external load is connected, it is negligable and stable if the load is the correct 75R.

I have also run several video outputs through 75R terminators from a single op-amp output on broadcast designs. Along with the op-amps mentioned by Jeff, the Linear Tech LT12XX series among others can provide excellent isolation with nearly immeasurable effect from one output to another. This type of output was considered for this design, but the slight advantages in no way outweighed the cost increase.

The D/A section is effectively 10 bit, and will achieve a better than 56dB snr with the post filter as described on the schematic. Using the current 2.32K for R34, the D/A uses 942 steps for the entire video signal with sync. One of the current test modes on the unit allows the validation of the snr by applying a worst case bit pattern to the D/A.

Darryl

Re: New Aurora converter - help wanted with review
 

Thanks Darryl.
One thing I don't understand.
At peak white all bits are high (=3.3V). So there is 3.3V at base of Q1.
2.6V at its emitter. The sync tips are 2Volts lower this is 0.6V.
Why do you have 0.2 Volts there?

Kind regards,
Darius

Re: New Aurora converter - help wanted with review
 

Darryl said that peak white is at code 942, not 1023. This is approximately 3.3 * 942/1024 = 3V so the sync tip will be at 1V at the base. Around 0.3V at the emitter.

I'm not sure why Darryl chose white level at 942. In professional digital composite video, white level is 844. This is because the subcarrier in saturated colours rises well above white. 942 allows a good margin for signals that rise above white. This is good practice since clipped whites usually look nasty. I have put white level at 853 in one design because this gave black level at 256 which was convenient.

Re: New Aurora converter - help wanted with review
 

Ah, thanks I understand.

Darius

Re: New Aurora converter - help wanted with review
 

Hi,
It's sort of straight forward where the 942 comes from. As Jeff mentioned, 844 is the typical white point value. This is required for a normal composite signal since the chroma information can go much higher. When only a monochrome output is required, you can take advantage of more of the D/A range. Since 0-100IRE represents 16-235, or 220 steps inclusive in a standard ITU656 data stream, and using a 70/30 video/sync ratio, the sync occupies approximately 94 steps for a total of 314. Since all the interpolation math is done at 12 bits precision, I round the results to the nearest whole integer multiple that best utilizes the 10 bit D/A range. This turns out to be a multiplier of 3 which results in the 3 * 314 = 942 steps. This yields 942/1024 * 3.3V = 3.03V range out of the D/A. If I had to add composite chroma, this would need to be dropped to the more conventional 844. With the 942 number, it allows for almost 10IRE of super white in the video.
This digital range is then matched to the output stage using the set resistors R34 and R35. Between how the digital range and set resistors are calculated, you can control the gain and operating point.

I've included a couple of picts from the spectrum analyzer of the RF output.

Darryl

Re: New Aurora converter - help wanted with review
 

I should have spotted where 942 came from. I ran through a few possible calculations but I didn't think of multiplying by 3.

The headroom above 942 also allows for setup (pedestal) if required.

The spectrum displays look very much as I remember them. The odd harmonics are very obvious all the way to 500MHz (max frequency for my spectrum analyser). The carriers look like square waves on a wide band scope.

Re: New Aurora converter - help wanted with review
 

Hi Darryl,
thanks for the pics. Looks equal to my output. Now I am sure that everything is ok.
To get the full dynamic range in the MK III converter, I removed the syncs from the video signal and added the new sync at the output. This gives 30% more "grey steps".

Kind regards,
Darius

Re: New Aurora converter - help wanted with review
 

That's a great way to get more dynamic range. Why bother processing the sync signals if you don't have to.

Darryl