Laser technology

[Sonnie]

I've often wondered exactly how some laser can pick up an audio and video signal and actually transfer it to sound and picture. Sounds like it would be complicated. Can anyone explain it in layman terms?

[drf]

Yes, but it'll have to be after work as I am running late.

[tweeksound]

I can comment on the digital audio side of things better than I can
on the video side.
I actually am teaching a "digital audio fundamentals" class tomorrow,
so this Will be good brush up material.

I am eager as well, to hear drf's post on the subject
as I have a fairly basic understanding of mainly the audio
aspect and much less the video aspect.

Here goes...

Sound, when in the air is mechanical or acoustical energy.
A vibrating apparatus basically vibrates the air by compressing the air
molecules (or any matter molecules) creating high pressure zones called
compressions. This is represented on our graphical waveform displays as a crest or the
uprising slope.
Since air wants to remain at the pressure level it was at before it was compressed these high
pressure zones bounce back or refract into equal and opposite low pressure zones ( the equal and opposite
low pressure zone is less equaled proportionately according to outside forces such as
friction, etc.) called refractions.

low pressure zones are represented on our graphical waveform displays as a trough or the
downward slope.

When this acoustic or mechanical energy meets with the diaphragm of a microphone it moves the diaphragm the same way. Compressions push the diaphragm in and refractions pull it back out. Somewhere in between these two extremes the
diaphragm sits at rest suspended in a homeostasis.

Assuming the microphone is of the dynamic type (for simplicity) the diaphragm has attached to it a spool of copper wire called a voice coil.
This voice coil sits within a magnet called a permanent magnet. The magnet doesn't move but the voice coil moves along with the diaphragm as
they are attached.

Because there is a magnet with a coil of wire suspended within it, there exists an electro magnetic field.
When the diaphragm moves and causes the voice coil to move within the boundary of the magnet, it causes fluctuation in the electromagnetic field

These fluctuations cause extremely minute and constant changes in the voltage created in the electromagnetic field which travel down the two ends
of the voice coil wire to an amplifier. This signal is now analogue. Meaning it's an electrical signal that represents the acoustic signal theoretically exactly.

The amplifier multiplies the small incoming voltage over and over until it's loud enough to be used.
For recording you only need a preamplifier to get the signal up to line level.
For PA systems you need a preamplifier and then a power amplifier to get the signal strong enough to
be heard well in a large area.

Let's assume we are recording the signal so we only need a preamplifier.
The preamplifier outputs a replica of the signal created in the microphones electromagnetic field
only at a higher voltage. 0 VRMS = no movement of the diaphragm and 1.228 VRMS is maximum movement
of the diaphragm.


When recording the analogue voltage from the preamplifier's output is first sent into an analogue
to digital converter (ADC, A/D).
This is a circuit that assigns a number to each voltage value.

For simplicity I will explain it as an 8 bit system. An 8 bit system is able to have 8 1's or 8 0's
and any combination of them, and there are 256 combinations.
Therefor an 8 bit ADC will allot the voltage into 256 levels including 0 V.
There are linear converters and there are logarithmic converters. Audio recording uses mostly logarithmic converters
meaning more of our 256 plot points are used from 0 - .614 VRMS than from .614 - 1.228 VRMS.
This allows lower level but equally complex passages of music to have more resolution than in a linear
converter system.

Cds are actually 16 bit and have 65,536 possible combinations of 1 and 0s.

Now, looking at our graphical waveform we have our vertical axis (amplitude) covered but the horizontal
axis (time) is not.

Just as as your favorite song might be 3 minutes and 45 seconds long, all audio has a dimension of time.
The horizontal axis represents time. And the bit depth plot points that we've discussed need to be plotted at equal intervals of time
in order to have a linear playback. These intervals are determined by the "sample rate".
Sample rate (or sampling frequency) is just like it's sounds. The rate or frequency at which the incoming
analogue signal is sampled or plotted at. It's measure is samples per second or Hertz.
Cds have a set sampling frequency of 44,100 samples per second or 44.1Khz.

This mean that for every second your CD plays by, there are 44,100 equally spaced plot points that are allocated a
bit depth (in our example 0 - 255). That's every .0000226 seconds!

Now since faster vibrations cause faster compressions and refractions of air, which result in smaller wave
size, you need a fast sampling rate to catch all those quick changes.

Humans are able to hear roughly from 20 Hz up to 20,000 Hz. Sampling a 41 Hz bass guitar note on a CD is
no problem. You've got 1075 plot points that you can allocate to a single compression and refraction
of that note. Looking at that waveform you wouldn't even see the grade in between the plot points.
In fact, the absolute minimum sampling frequency must be twice the frequency of audio you hope to capture.
That's why Cds are at 44,100 sample. To capture up to 22,050 Hz.
But many instruments produce overtones that reach well above 20,000. These overtones are what makes the instrument unique.
They are the reason why a violin's B4 doesn't sound at all like a trumpet's B4.
A cymbal can produce up to 40,000Hz or more!
If you try to sample a 15 Khz overtone of an acoustic guitar on a Cd you only have 2.7 plot points
which you can allocate to a single cycle (compression and refraction) at that frequency.
Rather than looking like a rounded complex waveform it will look more angular and sharp.
Because of this, that frequency will sound fairly different on the Cd than in real life.
Sampling frequency and bit depth is a balance between storage and processor ability, and fidelity.
Many feel 44.1 Khz 16 bit it's a reasonable balance and some feel it is not.
DVD audio is capable of 96Khz and 24 bit. That's a frequency response that reaches 48KHz and a bit
depth that allows for 16,777,216 vertical plot points!

Well, I'm not exactly sure how the laser does it but during the writing process on a DVD or CD, the laser
makes pits on the thin aluminum foil layer in the disk. During playback the laser shines on the pits and
groves and is reflected back into a photo sensor. When the laser is directly reflected on a flat section of the foil,
the photo sensor receives the full strength of the laser and that represents a 0 and when it hit's a pit less light
is reflected and that is a 1. (or vice versa). Every second the digital clock in the player expects to see 706,600 pits or landings and over 3.3 trillion on a single CD! It translates them into the digital words that represent the plot points on our vertical and horizontal axis which creates our waveform.
A digital to analogue converter (DAC) does basically the opposite of the ADC and uses these points to produce the same varying voltage that created the points.

The varying voltage is amplified and sent to speakers which are almost exactly the same as dynamic microphones only in reverse and much bigger.

The voltage travels thru the 2 copper wires that wrap around a tube to create the voice coil. The voice coil sits inside of a much bigger magnet. When the voice coil receives the varying voltages it changes it's electromagnetic charge and causes it to move forward or backward between the 2 poles of the magnet. The moving back and forth replicates as closely as possible the varying voltage
and the speaker's cone becomes a mechanical transducer and vibrates the air causing the same compression and refractions that were originally captured by the microphone.

At least that's how I understand it to work.
I'd love to learn more about how the laser writes and digital video.

[Sonnie]

I asked for it didn't I. Wow! That's a lot to swallow there. It's hard for me to comprehend numbers like 3.3 trillion.

[drf]

O.k in real laymens terms:

in normal audio, the electrical voltage varies over time. this variation in voltage is an exact (close enough) match to the sound pressure waves you hear. When this electrical signal is converted to digital, for any one given point instead of recording a voltage, it will record 2 Bytes of I and O's that represent that voltage.

All a CD player does is read the I and O's and then create the appropriate voltage. It does this with an internal clock so that the created voltage is proportionate to the original sound.

I hope that makes sense. There is of course alot more to as tweeksound has shown.

or did you mean the actual laser technology itself?

[Sonnie]

Actually I don't know how a laser works either, but more specifically I was wondering about how the laser reads what it does.

It's still hard for me to work through it, being I've never been much on physics.

[drf]

The CD or DVD is like a mirror, clear glass or plastic with a reflective backing. On a cd or dvd there is a concentric line of holes (reflective backing removed). When the cd/dvd spins the laser beam hits the surface and reflects back to a sensor, when it encounters a hole the beam stops reflecting. So the end result is that the sensor turns on and off in response to where the holes are located on the disc.

This stream of on/off signal is what the processor will decode (DAC) and produce the audio signal.

The stream of on/off signal also contains information that the cd player requires in order to know how many tracks there are on the cd and what location on the disc each track starts at.


As ar as lasers go:

A laser is a Light Amplification by Stimulated Emission of Radiation. Wikipedia has a section on lasers.

[Sonnie]

That's something new to me there... I had no idea there was an acronym for laser.

[tonyvdb]

My brain hurts after reading all this but thanks thats great information.

[thxgoon]

A laser is just a stream of light where all of the 'light' is traveling in the same direction. That's as layman as I can think to put it. IOW - the light doesn't spread out as it travels and is of a single wavelength meaning only one 'color'.

If it helps you can think of it in terms of an oly vinyl record player. On a record, pivots and hills move a needle up and down creating an alternating current that ultimately gets amplified and forces your speakers to move in exactly the same fashion. On a CD or DVD pits diffuse laser light so that it doesn't get reflected back and no current is passed on. The electronics read this as a digital zero. If it is reflected, a sensor turns that light into an electric current which the electronics read as a 1. If you had a stream of 100 1's in a row though, the electronics would just get a continuous current, so it has to have some way of knowing how many 1's there were in that stream. For this they put a clock which is precisely calibrated for the speed at which they expect 'bits' (1's and 0's) to be coming in. From there the Digital to Analog (D/A) converter takes over and creates an analog signal just like the one that would have come out of your record player. This all happens blindingly fast....

On a CD, the 'words' are written with 16 1's and zeros, and they are read at a rate of 44,100 words per second. Per channel!!! That's over a million 1's and 0's processed every second just for 2 channel music. Now imagine how many are processed on Blue Ray and HDDVD.

Hope that helps a little.

[lcaillo]

Everything you could ever want to know about lasers, and everything else about electronics can be found here:

http://www.laserfaq.org/sam/odfaq.htm

and

http://repairfaq.ece.drexel.edu/sam/lasersam.htm

and in general about electronics

http://www.laserfaq.org/sam/samsites.htm