Low frequency performance.

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Low frequency performance.

Postby Steve Anderson » Fri Jun 01, 2007 9:23 pm

I have started this thread here rather in the Images & Sound section as it seems more appropriate.

Attached is another .wav file I created in software, it's a simple 12.5Hz square wave with the NBTV sync thrown in. (missing included).

Playing it back on the (very cheap) soundcard/motherboard here resulted in the waveform shown in the .jpg. Which was far better from a LF point of view than I ever expected. It's 'almost' DC coupled. It's of a complete frame of NBTV, you can see the missing sync.

I created two other files, one of complete black with just the syncs, and the same with white. There was a shift in the sync tips between these two files, but not like you would expect with AC coupling.

Either I just have hit lucky, or perhaps soundcards are generally like this?

Of course this is just the playback side, record section has yet to be used.

Steve A.

Thinking about it further, the 'almost' factor could be due to power supply regulation, there was no real load on the output, just the 10 megohm scope. But who knows how the D-A is configured?

The fact that the white part of the waveform is ruler straight (ignoring the overshoots) has got me quizzical.
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DC Coupled.jpg
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Postby AncientBrit » Fri Jun 01, 2007 10:31 pm

Pretty impressive Steve.

I'm not sure about the small tilt on one edge.

Like you, any LF tilts I've seen have been symmetrical about leading and trailing edges of the waveform.

Intriguing...


Regards,


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Postby AncientBrit » Fri Jun 01, 2007 10:34 pm

Looking more closely I note that the sync tips exhibit the usual "CR" charge/discharge shape.

It's only the PW top that's flat.

Is it possible that you are driving the card into PW clipping?

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Odd behaviour

Postby Steve Anderson » Sat Jun 02, 2007 1:56 pm

Yes, it's all a little intriguing. The reason I created the first file (in the Images and Sound section) was for this very reason. It's a full amplitude 1kHz sine wave, 8-bit data sampled at 48kHz in mono.

Without altering any settings the harmonics were around the -45db mark which is about right for 8-bit audio, although not perfect. I would post the FFT analysis if I knew how to convert the data from the spectrum analyser. Another task, another day.

What I might do is build an external D-A that occupies a memory address and mimics a 'real' soundcard.

However, in the context of NBTV the existing arrangement is plenty good enough. There should be no problem with stripping out the syncs (for once) and I doubt that the artifacts would be noticable. After all, this is rather an extreme waveform pushing the format to its limits, and the on-board sound card is cheap and dirty.

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Postby Klaas Robers » Sat Jun 02, 2007 9:01 pm

Steve, try to connect a 10k resistor at the output and look again at the wave form. The wave may show then some sag.

Read my article about "Sag" in NBTV in Newsletter Vol.27 No.2.
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Output drive.

Postby Steve Anderson » Sat Jun 02, 2007 9:42 pm

Klaas Robers wrote:Steve, try to connect a 10k resistor at the output and look again at the wave form. The wave may show then some sag.


Nope, no difference at all. With a 1k load, also no difference whatsoever. At 100 ohm there was some clipping in the output, but that doesn't suprise me as there's probably some current limiting in the output stage. This is a line-out stage, not designed to drive headphones or speakers.

The output waveform is about 6Vp/p, so current limiting at 100 ohm comes as no suprise.

What I suspect is the reference Voltage for the D-A is being influenced by the output, with normal audio it wouldn't be a problem, but here we're dealing with frequencies far lower than we can hear, so the soundcard section of the motherboard is simply optimised for audio, not NBTV.

The fact that it is as good as it is came as a suprise to me.

Perhaps that's why the bass in the jazz I listen to on-line is so good.

Steve A.
Last edited by Steve Anderson on Sat Jun 02, 2007 10:38 pm, edited 3 times in total.
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Postby Klaas Robers » Sat Jun 02, 2007 10:06 pm

Then you have a well designed sound "card". On the other hand, it is not that difficult to make a DAC that is DC coupled to the output. You could make a .WAV file to try how low frequency your card goes.

The better CD-players are designed to reproduce 2Hz to 20kHz within say 1dB or even 0,1dB (1cB, one centibell) at their line output. This is good enough for NBTV, but if it was 20Hz tot 20kHz within 3dB this would cause visible sag. It is THE problem for cassette recorders, even reel tot reel recorders. Happily we have CD-R nowadays.

CAUTION !!! Never try to record DC signals onto CD-R. The coding of CD is designed for audio only. So signals close to zero (silence) can be recorded and/or signals that are (wildly) changing. If you try to record a constant DC-level the play-back of the CD might go "bananas".

For NBTV this is less a problem, because the right channel (sound) confirms to the audio definition and solves the encoding problem in most cases.

For CD-ROM, which might contain long strings of identical bytes, this problem is overcome by "scrambling" the byte stream with a known pseudo random scrambling stream. At play-back descambling is done by the CD-ROM-drive. This explains that when you run a CD-ROM in your CD-audio player that you hear a loud and agressive noise, if this is not wisely muted by your player.
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Postby Steve Anderson » Sat Jun 02, 2007 11:00 pm

Klaas Robers wrote:It is THE problem for cassette recorders, even reel tot reel recorders. Happily we have CD-R nowadays.


Somewhere in another thread I mentioned I was trying to get a hold of a couple of virtually unused Otari open-reel quarter-inch recorders. I did some experiments with Telefunken machines in the 80s in pushing their response beyond what they were designed for. The concept was to record FM data at a frequency beyond the heads extinguishing frequency.

The Telefunken machines could play back their own bias of 160kHz or so! So I still feel there's a purpose in doing this, though I do admit I'll probably be the only one doing it!

Now if that's not in the spirit of mechanical TV, I don't know what is! Open reels, rotating mechanical parts, Andrew should be drooling!

I attach a .pdf file about early tape recording for those that are interested. Don't forget that it's going to cost you four shillings and sixpence!

Steve A.
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NBTV wire recording.

Postby Stephen » Sat Jun 02, 2007 11:57 pm

Steve Anderson wrote:Now if that's not in the spirit of mechanical TV, I don't know what is! Open reels, rotating mechanical parts, Andrew should be drooling!
I remember when there were combination gramophone/wire recorders. The rim of the gramophone turntable served as the take-up spool for the recording wire.

It would be fun to have a NBTV wire recorder wherein a hub on the scanning disc drive shaft serves as the wire take-up spool. That would be 1930s style technology!
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Related topic.

Postby Steve Anderson » Sun Jun 03, 2007 1:29 pm

Most of the more 'mature' members here will know all about Vera, a .pdf all about her is attached.

Again, she's got reels, rotating parts and moves really fast! "Fnarr! Fnarr!"...be quiet Finbarr Saunders! I believe that she was the first practical(?) video tape recorder, but I'm not sure about that. The BBC do refer in the introduction to the Ampex system which I presume came to be the Quadraplex format using 2" tape.

I hadn't read it in some years, but the principal used for recording the video track I had thought about in the past. I started and built the filters which worked fine.

I split the incoming signal into two bands, one from DC to 50Hz, the other from 50Hz upwards. The low frequency band then FM modulates a 10kHz carrier. The two signals then are recorded in a standard stereo manner.

On playback, the FM track is demodulated and mixed in with the other track restoring the original signal.

You lose the use of the second track for sound, but there might be a way around that.

My concern was the delay in the FM modulation/demodulation process, but it seems that the BBC got around this, but they don't give a clue as to how they did it. One method is to have the FM playback head slightly in advance of the HF head, but with the recording systems we use that's not an option.

The alternative is to delay the HF channel by the same amount. Today this would be done in the digital domain. But trying to keep things simple I considered the use of analogue bucket-brigade delay lines, the Philips TDA1022 for example. Suppliers? Nil. Unless you use those 'obsolete component' sharks.

With the frequencies the BBC were using they could have used a delay line comprised of correctly terminated coax cable. Broadcasters use 75 ohm coax cable which with a velocity ratio of 70% gives a delay of 5nS/m. For our use I imagine it would be kilometres long! Plus the losses and equalization required would be excessive.

So for the time being it's been shelved.

Steve A.
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Re: NBTV wire recording.

Postby Viewmaster » Sun Jun 03, 2007 6:37 pm

Stephen wrote:It would be fun to have a NBTV wire recorder wherein a hub on the scanning disc drive shaft serves as the wire take-up spool. That would be 1930s style technology!


I like it! But at 12 1/2 revs per sec disc speed the wire take up spool would only be approx 5/8 inches dia. as wire speeds were 24 inches per sec. The motor would have to be larger as would the belt drive to take the extra load but what a real old fashioned system that would be.

Great web site here about wire for anyone about to build this NBWTV
(Narrow Band Wire Television!)...
Includes an actual recording too.
http://www.videointerchange.com/wire_recorder1.htm
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Re: NBTV wire recording.

Postby Steve Anderson » Sun Jun 03, 2007 7:50 pm

I like it! But at 12 1/2 revs per sec disc speed the wire take up spool would only be approx 5/8 inches dia. as wire speeds were 24 inches per sec.


Do you mean there was actually a standard? I find that amazing! I guess the tape recording industry eventually settled on speeds that were a factor of two from each other to preserve what sanity was left. Even the Compact Cassette inherits its 4.76cm/s speed from open reel recorders.

30 ips (inches per second) = 76cm/s, Studio mastering.
15 ips = 38cm/s, also Studio Mastering and other high-quality recordings.
7.5 ips = 19cm/s, general-purpose broadcast use, high-end home use.
3.75 ips = 9.5cm/s, perhaps the most popular speed for home use.
1.875 ips = 4.8cm/s, voice quality tape messages, dictation and Cassettes.
15/16 ips = 2.38cm/s, voice logging for Air Traffic Control, Police etc.

There have been many, sometimes bizzare, incarnations of audio tape formats. Marantz in the 1970s launched a range of dual-speed Hi-Fi Cassette recorders that used standard Cassettes you could buy anywhere, but ran at 3.75 ips (9.5cm/s). The El-Cassette was the same idea, but using quarter-inch tape. Thankfully they were failures, but I admire the fact they tried.

Steve A.

I have to admit I really did seriously consider buying one of those Marantz machines, but I didn't have the cash at the time.

It also brings to mind 16rpm records. These perhaps were the original 'floppy', for that's what they were! In the 60s I was learning Japanese and the lessons came on a record that was simply a stamped piece of plastic, they were as flexible as a magazine cover, were usually red and you could see through them. I remember my father had to shop around to find a record player that had 16rpm on it.
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Back on track.

Postby Steve Anderson » Sun Jun 03, 2007 10:25 pm

Anyway, notwithstanding the above, let's return to the subject at hand.

Somewhere I have seen reference to using the printer/parallel/LPT port as an output. Possibly an input too. This would require quite simple external circuitry to generate a DC coupled NBTV waveform, and a suitable A-D for recording.

Using an external clock to give the PC a prod would ensure that timing is not influenced by OS overheads. Anyone tried this at 48kHz?

Interrupts would be the most hygenic way to go rather than polling.

Steve A.
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Re: NBTV wire recording.

Postby Viewmaster » Sun Jun 03, 2007 10:42 pm

Steve Anderson wrote:
I like it! But at 12 1/2 revs per sec disc speed the wire take up spool would only be approx 5/8 inches dia. as wire speeds were 24 inches per sec.


Do you mean there was actually a standard? I find that amazing! I guess the tape recording industry eventually settled on speeds that were a factor of two from each other to preserve what sanity was left. Even the Compact Cassette inherits its 4.76cm/s speed from open reel recorders.


Yes,seems there was. That URL I gave says so and Wikipedia bears this out too...
http://en.wikipedia.org/wiki/Wire_recording

The reels had to be large in order to get a fairly constant wire speed throughout the recording.....no capstan is used. An oscillating cam also used to layer the wire correctly......more power required from the disc motor....how about two wire recs side by side, one for vision and other for sound? :)

quote from Wiki...
"Compared to later tape recorders, wire recording devices had a high media speed, made necessary because of the use of the solid metal medium. The wire reels were recorded or listened at nominally 24 inches per second (610 mm/s), making a typical one-hour reel 7,200 feet (approx. 2195 m) long. This enormous length was possible on a spool of under 3 inches in diameter because the wire was nearly as fine as hair. Since the wire was pulled past the head by the take up spool, the wire speed increased as the diameter of the spool increased."

There's a USA 1930's wire recorder for sale on eBay at present...standing at £5 so far.

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Mr. Baird's hard disc drive.

Postby Stephen » Mon Jun 04, 2007 12:16 am

John Logie Baird proposed using magnetic recording for video signals in early 1927. His British Patent 292,632, filed 26 January 1927, illustrates a multi-disc hard disc drive with separate record, play and erase heads for each disc! The play heads could reproduce at a faster rate than the record heads.

Mr. Baird's primary focus with such a scheme in 1927 was to overcome the sluggish response of photocells in order to provide higher definition pictures. What he had in mind in the patent was to scan three different portions of a picture simultaneously at a low rate whilst reproducing the entire recorded picture at three times the rate.

For instance, the scheme could simultaneously record three 30 line portions of a frame simultaneously whilst reading each complete 90 "high definition" picture for transmission. The erase heads would erase each frame after the read heads read the complete frame to allow recording a subsequent frame. He does mention that a similar arrangement might simply record a programme for later transmission.

In any case, Mr. Baird's 1927 proposal is much like a modern hard disc drive. There is a copy of this patent in the "Patents and Articles" section of the forum.
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