h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

On some (now very old) mainframe computers, a byte is 6 bits. This is probably irrelevant nowadays, and I've never come across such a machine myself.

More seriously, the byte is not the fundamental unit of information theory - that accolade goes to the bit.

Also, I heard that baud doesn't always equal bps, but I don't know the details.

I seem to recall that the strict definition of baud is in terms of 'information'. This would exclude error correction and other control information. However, I wasn't aware that this was actually used.

Hi,

I enjoyed your entry very much. I especially liked the section on decibels and logarithmic scales.

I do have a comment about baud though. It is not strictly a measure of bits per second. It is more accurately described as the number of signaling elements per unit of time. Historically they were once one and the same. The signaling rate is limited by the bandwidth of a channel and its signal-to-noise ratio. But engineers found ways of packing more than one bit into a signaling element.

Imagine I had a flag with a one and another with a zero. If I could only raise and lower them at a rate of one per second, I would be transmitting at 1 baud. If I made up four flags (00, 01, 10 and 11)I could then transmit at 2 bps still using a baud rate of 1.

The techniques used are phase shifting ( say 0, 90, 180, 270 for each of the above bit combinations), vestigial sideband modulation (uses four different amplitudes of an unused sideband) and quadrature amplitude modulation (a mixture of phase shifting and different amplitude levels giving 16 combinations).

Also, a serial capacity expressed in bits per second doesn't translate to 1/8 the number of bytes per second. There's usually a stop bit and parity bit too. So you can divide by ten.

Ausnahmsweise, wie üblich.

P.S. My real name is that of an obsolete unit.

Pete (Keeper of Disambiguating Semicolons) said:
> On some (now very old) mainframe computers, a byte is 6 bits. This
> is probably irrelevant nowadays, and I've never come across such a
> machine myself.

You are right though. Some older machines had byte sizes that were not 8 bits. If I remember correctly, PDPs had 9 bit bytes. It's only been recently (historically speaking) that 8 bit bytes are standard. I don't know why, though; convenience, presumably.

--
Rebus

Well, let's hope that those days are really over. It's unfortunate enough that eg 'word' always mean something different, that 1 megabyte is sometimes 1000,000 bytes and sometimes 1024^2 bytes and so on and so forth ...

Bit rate is a measure of the amount of information passed. Baud rate is a measure of the speed of digits transmitted to line. If there no complex coding , no error bits etc, i.e. nothing added, then baud rate = bit rate.

Modems sometimes use coding systems to improve transmission quality. e.g. by using ternary digits i.e 3 elements (+,0,-) instead of the bits two (0,1).This bipolar code reduces the line frequency and thereby reduces the loss of signal power.