Line-out vs earphone?

[Steve Anderson]

Now, was that 200mV peak, or p-p...?

There you go. When it comes to PC related stuff the definitions are further blurred, p/p, peak, or heavens forbid a sane RMS value?

The best bet is to assume the worst, it will be inverted, or maybe not. It will be only 100mV p/p or 10V p/p, make your gear able to accept all, it is actually quite easy these days.

Steve A.

[M3DVQ]

Now, was that 200mV peak, or p-p...?

There you go. When it comes to PC related stuff the definitions are further blurred, p/p, peak, or heavens forbid a sane RMS value?

Well this isn't from any kind of specification. This will be what I've found experimentally. I think it was around 200mV peak, into a high impedance which was what I was building at the time. RMS isn't much use when you're talking about signals that you want to put through amplifiers is it? You will want to know what the peak instantaneous voltage will be to ensure you don't clip the signal in your amplifiers.

I agree that it's best to design the input stage of your circuit around the source you are going to be using, or better still design it to work with any signal level (within reason) if it's not too much extra work/cost to do that.

What one could do is build circuits to take a 1V input as per the nbtva specifications, but have an extra input stage you can switch in to bring inputs from a computer headphone or line output up to the right level.
(this is also how I handle inverted signals, I have an inverting buffer stage that can be jumpered in and out of the input signal path)

[Klaas Robers]

Of course I come from a time long ago, when CD was conceived. After that everything was changed, in a negative way of course, people even listen to MP3 today. Awfull......

But when CD started there was a polarity defined and there was a level defined for line outputs. The polarity was positive for positive numbers on the disc. So #7FFF was the maximal positive voltage, #0000 was zero, and #8000 was maximal negative. Coding on CD is in two's complement.

In that time (1980) the audio standard output for tape recorders, even cassette recorders was about 500mV rms for the line output. CD-players should be compatible with that. However as tape is rather tolerant on overloading signals, much better than CD, a comfortable headroom of 12 dB for CD was assumed. This is because overload of the A/D and D/A digital system introduces very audible distortion, so should be avoided under all circumstances.

Thus the real peak output of a CD player was designed to be 2 volt RMS for pure sine waves, that is 2.8 volt peak. I have several CD-players from 1983 to 1995 and they all give that output voltages on a test CD that I made and that contains a sine wave from #7FFF (= + 2.8V) to #8001 (= -2.8V).

The club CDs I made to output 1 volt black to white white and 0.4 volt sync pulses, so 1.4 volt peak to peak, if white is present in the picture. I have checked that at the line outputs of my vintage players.

Nowadays earphone outputs should be volume controlled of course and the polarity is of no sense as earphones are not standardised in any way on that aspect. I can also understand that these earphones might cause ear damage if you feed them with 2 volts RMS. So the output voltage is always less than this nominal line output signal.

The 1 volt was a value that was already a club advice, although it was unclear if that was including or excluding sync pulses. The same problem occured in CCIR systems, where I have seen signals of 1 volt black to white with an extra 0.4 volt for the syncpulses, and also 0.7 volt black to white with an extra 0.3 volt for syncpulses, as well as 0.7 volt video including the sync pulses. What is in a standard.....

[Steve Anderson]

The same problem occured in CCIR systems, where I have seen signals of 1 volt black to white with an extra 0.4 volt for the syncpulses, and also 0.7 volt black to white with an extra 0.3 volt for syncpulses, as well as 0.7 volt video including the sync pulses. What is in a standard.....

All professional video equipment has analogue composite inputs and outputs standardized as 1.0V from sync-tips to peak white. This is composed of 300mV of sync and 700mV of active video. 100% saturated colour bars extends the sub-carrier value (PAL or NTSC) beyond 1.0V and below black level, but not lower than the sync-tips.

This is in a terminated 75Ω circuit, i.e. the source output has a source Z of 75Ω and the destination equipment has an input Z of also 75Ω. The 1.0v specification is the actual voltage within the coaxial cable joining to two pieces of equipment which should be of the same impedance.

The consequence of this is that the output stage of the source equipment is acually generating 2.0V p,p. Much like the GPO 600Ω audio specification (now long defunkt).

Enter analogue RGB...the two most common 'standards are 700mV video on R and B, with the syncs added to the G signal making it 1.0V. There is of course no sub-carrier. This is common with US manufacturers.

The second 'standard' is actually split into two sub-standards, RGBS and RGBHV. The R,G and B signals are all 700mV and the syncs (S) carried on a forth 75Ω coax at anywhere between 2.0 and 4.0V, odd but there it is.

RGBHV adds a fifth coax and the Horizontal and Vertical syncs are carried separately, much like an analogue VGA PC monitor.

These 'standards' are rarely seen these days and all now carried on a single coax in digital formats with multiple audio tracks multiplex in at 270Mb/s (625 SDI), 1Gb/s or 2Gb/s for HD depending on the standard.

Then there is anlogue YUV, with a luminance signal (Y) and two colour-difference signals (U and V). The Y signal is no different to an analoue monochrome signal, which is what it is, complete with syncs. I'm not going to go into the colour-differencing here, but if interested here's a site that covers and explains all analogue formats for TV worldwide...be prepared to spend some time there...

http://www.pembers.freeserve.co.uk/World-TV-Standards/

Returning to NBTV, there's no requirement at normal NBTV frequencies to use terminated circuits, just screened leads as per audio gear. So I guess a long time ago 1.0V was chosen for NBTV as an inheritance from FSTV, but without the 75Ω bit.

Also of note, the same 1.0V/75Ω standard is the same for domestic analogue video gear, VHS, DVD etc. shame they never got their act together for the audio.

Steve A.

P.S. Analogue video is never inverted, even in domestic gear, they at least got that right!

[nbtv]

I am now getting very good pictures with an earphone output from a laptop. The sound-files need to be inverted in Audacity, but this is easy and quick.

Thanks for the feedback.

[Klaas Robers]

Steve,

I experienced in the Black and White TV time VBS video signals that were 1.4 volt peak to peak when terminated into 75 ohm. VB signals were 1 volt black to white.

As well I experienced that the CVBS signals to a TV via the SCART connector is 0.7 volt peak to peak, also terminated into 75 ohm. The SCART RGB signals are also 0.7 volt when terminated into 75 ohm.

I agree that for NBTV termination is not needed.

[Steve Anderson]

Steve, I agree that for NBTV termination is not needed.

Agreed, but when one starts to dabble in bandwidths of (say) 500kHz plus, then it is worth pondering the use of terminated lines. If they're short, say across a room (even of convention size), then maybe not as long as the source can cope with the cable capacitance. Stability with op-amps becomes an issue here, an output isolation resistor is usually all that's needed as long as it in conjunction with the cable capacitance doesn't lead to a premature HF roll-off.

But here were getting out of the realms of NBTV and into MDTV, say 120-lines at 25fps non-interlaced, this requires the best part of 300kHz of bandwidth. Even here, with short lengths I wouldn't bother, but it may be worth a few stabs at the calculator to confirm this. I have an easy access to 75Ω coax cables as left-overs from 'real work' projects, so I tend to use them.

I have to admit I have never seen 1.4V p/p analogue composite video, unless you're including the 100% bars peak sub-carrier value...which does come close to that figure...but what the heck, standards will always be broken and ignored by someone...two resistors and/or a pot will fix that...but since 1984, never seen it...

What I have seen plenty of is unterminated lines where the destination doesn't have a 75Ω input Z resulting in bleached-out highlights...

Steve A.