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56k modems relied on digital trunk lines

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Re: 56k modems relied on digital trunk lines

#111

Post by numpad0 »

IncreasePosts wrote: post #108
sneak wrote: post #89

Earlier quoted context omitted.

nyquist-shannon means that you only need to sample at 192khz if you need to encode signals up to 96khz. humans can’t hear above 20khz. adult humans can’t hear above 16khz or so, we lose the top end before age 20. this means that the standard 48khz sampling rate covers the entire human hearing range and then some (0-24khz). any sampling rate over 48khz for sound intended for human hearing is a total waste.

Why do 96 or 128khz sampled audio files sound better than 48khz ones? I blind tested and could always tell the difference between them, but not between 128 and 192

sampling theorem only applies to sine waves. the rate is bit like the order of (fourier)series expansion and so approximations deviate as rate reduces. how many orders is enough depends and is situational

Re: 56k modems relied on digital trunk lines

#112

Post by badc0ffee »

xnx wrote: post #7
sureIy wrote: post #5

That reminds me of my short time with ISDN and the amazing availability to receive calls on the second line without forcing me offline.

I'm still mad that they high-quality voice audio possible through ISDN didn't become ubiquitous. It's ridiculous to hear the clipped frequency range of a plain old telephone line during radio interviews in 2025.

I thought VoLTE was supposed to fix that, at least for mobile phones.

Re: 56k modems relied on digital trunk lines

#113

Post by badc0ffee »

phil21 wrote: post #34
stuff4ben wrote: post #6

Earlier quoted context omitted.

That 128Kbps on ISDN was the like the gold standard back then! I knew some sysadmins who had that installed to their homes so they could be available at any time. All paid for by the company they worked for.

The most bang for buck "employee benefit" I ever offered to my guys back in the earl 00's was a T1 line to their home for free. We could do this since local loops to most folks were about $150-200/mo, and we already had a channelized DS3 terminated at our rack at a local datacenter for our phone banks. If you bought your own DS1 retail you'd be paying upwards of $1k/mo back then to a provider. It was by far the best…

I had 1.5 Mbps DSL in 1999, and I think nearly all of my co-workers either had that or a cable modem with similar speed. I think T1s were like 5x the price for the same download speed.

Re: 56k modems relied on digital trunk lines

#114

Post by joquarky »

stuff4ben wrote: post #6
sureIy wrote: post #5

That reminds me of my short time with ISDN and the amazing availability to receive calls on the second line without forcing me offline.

That 128Kbps on ISDN was the like the gold standard back then! I knew some sysadmins who had that installed to their homes so they could be available at any time. All paid for by the company they worked for.

I had one for work as well in the late 90s.

The other (often overlooked) benefit that ISDN provided was 24/7 connectivity in an age of dialup.

Oh and you could spoof your outgoing phone number for caller ID ]:D

Re: 56k modems relied on digital trunk lines

#116

Post by timthorn »

iptel wrote: post #40
plorg wrote: post #39

Earlier quoted context omitted.

Revealing my ignorance here, but was (is?) there a telephone equivalent of anycast such that, say, the 1-800-... Or 1-900... numbers would be routed differently based on location? My basic knowledge of phone systems suggests it would at least be possible.

Absolutely, when call hits local switch it can be terminated differently based on its location. Particularly pertinent for modem based services in the 90s. A single national dialup number would terminate on 100s of local pops and routing decisions would be done to keep traffic as local as possible.

Internet Thruway from Nortel allowed multiple ISPs to use the same local termination hardware; If I remember the details correctly, different national numbers could be terminated on the same box, with the subsequent IP traffic routed to the correct ISP.

Re: 56k modems relied on digital trunk lines

#117

Post by kdmtctl »

plorg wrote: post #39
OhMeadhbh wrote: post #17

Yup. I worked on the "Rapport" series of switches at Bell Canada. It was DS1 (Digital Signal 1) out one end and a rack full of Zyxel modems on the other side. The idea was RBOCs (Regional Bell Operating Companies) would put these in their CO (Central Office) and terminate 56k modem signals over the analog "last mile" loop to the customer premises and then do Frame Relay over the phone company's data lines to your ISP…

Revealing my ignorance here, but was (is?) there a telephone equivalent of anycast such that, say, the 1-800-... Or 1-900... numbers would be routed differently based on location? My basic knowledge of phone systems suggests it would at least be possible.

Not in the US, but we had a country-wide number that each local station simply routed to a local extension. Sometimes, even multi-line wasn’t available at some stations, so it was just a sequential list of extensions to try. Looks like a hack, but it worked flawlessly.

Re: 56k modems relied on digital trunk lines

#118

Post by sneak »

IncreasePosts wrote: post #108
sneak wrote: post #89

Earlier quoted context omitted.

nyquist-shannon means that you only need to sample at 192khz if you need to encode signals up to 96khz. humans can’t hear above 20khz. adult humans can’t hear above 16khz or so, we lose the top end before age 20. this means that the standard 48khz sampling rate covers the entire human hearing range and then some (0-24khz). any sampling rate over 48khz for sound intended for human hearing is a total waste.

Why do 96 or 128khz sampled audio files sound better than 48khz ones? I blind tested and could always tell the difference between them, but not between 128 and 192

Did you generate the 48khz ones from the 96/128khz ones via downsampling or are they different recordings?

Are they the exact same volume? We perceive things slightly louder as higher quality.

Is it a double blind test, ie an ABX test?

Are the bit depths the same? Many 96khz sampled files use 24 bits per sample, whereas 48khz usually uses 16 bits per sample.

Re: 56k modems relied on digital trunk lines

#119

Post by [deleted] »

numpad0 wrote: post #111
IncreasePosts wrote: post #108

Earlier quoted context omitted.

Why do 96 or 128khz sampled audio files sound better than 48khz ones? I blind tested and could always tell the difference between them, but not between 128 and 192

sampling theorem only applies to sine waves. the rate is bit like the order of (fourier)series expansion and so approximations deviate as rate reduces. how many orders is enough depends and is situational

[deleted]

Re: 56k modems relied on digital trunk lines

#120

Post by chipsa »

immibis wrote: post #2

Isn't it the opposite of the headline? Modems were hamstrung by digital phone lines you didn't know we had. One final trick allowed them to match, but not exceed, those digital lines.

The analog lines before digitalization were limited to 8kHz bandwidth. This is because to deliver a circuit between two points, the system generally figured out which bit of bandwidth wasn’t in use and frequency shifted the entire connection into that band.
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