CD line out vs headphone out

[Andrew Davie]

I'm aware this topic has been discussed before, but I'd just like to open further discussion on the validity and/or consequences of using a headphone-out feed from a portable CD player vs. a line-out feed. I've had the chance to do a couple of oscilloscope captures of both and I'm fairly sure both would give an acceptable NBTV image.

These images were recorded at the signal-in of the LED driver circuit board while track #5 (white screen) was playing on the CD.

Comments?

[Steve Anderson]

Andrew,

There's no real difference as far as NBTV is concerned with perhaps the following exceptions....

Line out...

1) Usually of fixed level which from the same player should be reasonably consistant in level.

2) Moderate output impeadance, load with no less than (say) 50k.

Headphone out...

1) Variable level via volume control, easy to forget you've got it set to zero and can't figure out why you're not getting any signal.

2) Lower ouput impeadance, designed to drive headphones of around 32 ohms or less. Will drive any reasonable circuit easily.

3) (Very) slightly higher distortion, but so low it's not an issue for NBTV.

..that's about it really...

Steve A.

[Klaas Robers]

The real difference is that the earphone-out has no definition of the polarity. For the line-out this is defined and gives positive video from the club-CDs as well as from the files coming from Gary's programs.

The disadvantage of an earphone output is also that the amplitude is not fixed. You CAN make it the same as the line-out amplitude, as Andrew did for the oscillograms, but in practice the volume control does the same as the contrast pot in your video chain. I dislike two controls that do the same. With a line-out I can put my contrast knob to a well known setting (the fixation screw pointing upwards) and then I will get a reasonable picture from a CD. This then is the case for different CD-players.

In practice the output impedance (resistance) of a line-out is somewhere between 100 ohm and 1k. It can be loaded with a pot of 10k safely without real fall back of the output amplitude.

[Andrew Davie]

The real difference is that the earphone-out has no definition of the polarity. For the line-out this is defined and gives positive video from the club-CDs as well as from the files coming from Gary's programs.

Definition or not, the line-out sockets, on the two portable players that I have, have differing polarity! For anyone starting out, then, the safest thing is to measure the polarity/waveform and make no assumptions about "standards"!

Where I'm coming from is "what is the easiest way for a beginner to build a monitor?" And if you don't have to find a CD player with a line-out, then that's great.

[Klaas Robers]

The problem is Andrew, that you can't measure it if you have no oscilloscope at hand. The mean value of the output will always be zero, as the audio signal has no DC level. Always a capacitor in the output lead. That is the reason that clamping or DC-restoring is needed for video purposes.

When building an NBTV-monitor so many things are uncertain. The best thing you did is buying an oscilloscope. Only then you can SEE what you have got. But suppose that you are 14 years and you want to do something in NBTV?

The video world is used to a fixed polarity. But also there is no DC coupling. The DC value of video signals when sent over cables can be everything. In most of the cases the mean voltage is zero