24 Mar
2007
24 Mar
'07
6:57 p.m.
We will get several comments after this post...
I hope y'all will correct me.
The acoustic coupler on the old teletype was 1200 baud, 10 characters per
sec. to match the machine. It was FSK (Frequency Shift Keying) with two
tones 1200hz and 2400hz for 0 and one. Depending on the # of start, stop
bits in the serial stream and parity baud .NE. bytes/sec.
Phone lines have a 2500hz bandwidth, and today's modems use QAM (Quadrature
Amplitude Modulation). You can visualize this if you think of a signal as a
rotating vector on an xy grid - say a constant sine at constant amplitude
would represent a circle.
put 4 points uniformly on the circle and XY grid, and you have 4 QAM. 16
points (rectangular array, not on the unit circle), 16-QAM etc. Each point
represents 2 bits in 4-QAM (dibits), and 4 bits in 16-QAM. this is called
the signal constellation.
To further encode the signal and reduce errors, they use techniques called
viterbi and trellis encoding, these limit the accepted signal transition
from one dibit, qbit to another.
In addition, modern modems train the channel. That's the white noise you
hear at the beginning of the dialup - both ends are doing equalization for
this connection. If errors occur, they fall back to lower data rates and
retrain.
Our 56K modems never get there here in the US, it is an FCC limit, and the
reasons I am not sure.
These basic principles above also apply to cell/radio transmission. There
are other goodies they can throw into the algorithms to reduce multipath
reflection (ghosts, as in TV) due to buildings objects, aircraft.
to conclude, Ill bet with a homemade cell phone acoustic coupler to -> RJ-45
jack interface you could get same performance as dialup, near 56K.
Needed:
Sponge/muffs to adapt the cell handset
electrostatic mic/speaker
some op amp stuff to get the signals leveled
resolve the dial-in and dial-out issues to fool the PC for dial tone and
auto answer (and interface to the cell keypad to answer and send)
Randy
From: Chris M <chrism3667 at yahoo.com>
Reply-To: "General Discussion: On-Topic and Off-Topic
Posts"<cctalk at classiccmp.org>
To: talk <cctalk at classiccmp.org>
Subject: pretty much OT: acoustic coupling
Date: Sat, 24 Mar 2007 13:39:38 -0700 (PDT)
my application anyway. What is the typical maximum
speed of such a device, be it antiquated (I actually
used to have one of the old Radio Shack units), or
home-brewed.
And lookee, here's what I had:
http://cgi.ebay.com/RADIO-SHACK-ACOUSTIC-COUPLER-MODEM-
TRS-80-AC-3_W0QQitemZ180098721480QQcategoryZ80229QQss
PageNameZWDVWQQrdZ1QQcmdZViewItem
In fact, that's an acoustic modem. What I'd like to
implement is a "bridge" if you will between a modem
and a cellular device that would allow connection to
the internet. Ingenious, no? ;)
Please, no one tell me about software and such that's
presently available on the market. I'm aware of it,
own 99% of it, and I'll say here and now it's flakey
at best.
The technology is antiquated enough, albeit not the
application. O woe is me :(
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24 Mar
24 Mar
9:10 p.m.
On Sat, 24 Mar 2007, Randy Dawson wrote:
We will get several comments after this post...
I hope y'all will correct me.
The acoustic coupler on the old teletype was 1200 baud, 10 characters per
1200 baud
??? You need less than a tenth of that for 10 chars per sec
to conclude, Ill bet with a homemade cell phone
acoustic coupler to -> RJ-45
???
D'ya mean a phone jack? That is RJ-11/RJ-14
25 Mar
25 Mar
12:16 a.m.
On 24/03/07, Randy Dawson <rdawson16 at hotmail.com> wrote:
to conclude, Ill bet with a homemade cell phone
acoustic coupler to -> RJ-45
jack interface you could get same performance as dialup, near 56K.
Sorry, but no. As a previos poster has pointed out, you'll hit the
codec (not G.711, but likely AMR for GSM phones, SMV for CDMA, or some
older codecs on TDMA systems), which tends to be speech specific, and
thus be really bad for any type of modulation coding. All POTS modems
assume a more-or-less linear channel.
You'll also hit against a few of Shannons' laws... in any case, you
can't get data through a channel at any rate higher than the lowest in
the chain - and the aforementioned codecs will give you decent speech
quality even down below 10 kbps. Furthermore, the CDMA codecs are
variable rate, since you're sharing badwidth with everyone else.
It comes down to that you really can't do better than using the data
mode of your handset. It's just that the codecs are so good the you
have the impression you're on a good line most of the time (and when
they fail, they degrade _really_ bad).
I've worked on the AMR-WB, VMR-WB and AMR-WB+ codecs (hello to anyone
from UdeS!) - it's a damn shame no one is actually
really using them
yet. The technology to have cell phones sound better than
landlines
(even at 14kbps and below) has been around for years now.
Joe.
PS. There is also a reason modems will never ever go above 56k. 56k
can only happen in the ideal case where you are really close to your
local office. At that point, the analog signal is digitized (u or
a-law) and sent over a 64kbps digital circuit - a slice of a T1 or T3
or OCwhatever. One of the bits is stolen for signaling - so your
bitrate is 7/8th of 64kbps: 56kbps. That is the slowest link in the
system to your "analog modem" ISP. And this will only work if the ISP
gets your circuit on a digital line (so the "analog modem" of the ISP
is actually fully digital) - the 56kbps modulation scheme does not
withstand a second D/A-A/D conversion.
Hooray for ADSL...
12:09 p.m.
This may be slightly OT but I've got an unused, new-in-box Radio Shack
acoustic coupler modem like the one in the eBay auction mentioned in
the original post that has been sitting around for entirely too long
(25+ years) so if anyone wants one, drop me a line. I ended up buying
a Hayes Smartmodem 300 that I used instead and never got around to
returning the RS modem. Whoops. Still has the original price tag on it
which is a constant reminder that my laziness has cost me a small
fortune over the years.
--
Golan Klinger <gklinger at gmail.com>
Dark is the suede that mows like a harvest.
6:59 p.m.
Golan Klinger wrote:
This may be slightly OT but I've got an unused,
new-in-box Radio Shack
acoustic coupler modem like the one in the eBay auction mentioned in
the original post that has been sitting around for entirely too long
(25+ years) so if anyone wants one, drop me a line. I ended up buying
a Hayes Smartmodem 300 that I used instead and never got around to
returning the RS modem. Whoops. Still has the original price tag on it
which is a constant reminder that my laziness has cost me a small
fortune over the years.
I think you better find a old phone for your modem.
All the modern phones I don't think will you work.
I remember trying to use style of modem years ago
and all the places I could call were long distance rates.
12:08 p.m.
On 24 Mar 2007 at 17:57, Randy Dawson wrote:
The acoustic coupler on the old teletype was 1200
baud, 10 characters per
sec. to match the machine. It was FSK (Frequency Shift Keying) with two
tones 1200hz and 2400hz for 0 and one. Depending on the # of start, stop
bits in the serial stream and parity baud .NE. bytes/sec.
The old ASR 32/33 modems were strictly analog devices. I doubt that
they could have responded at a 1200 baud rate. I thought that Bell
103 frequencies were used, but I'm not certain. The original
acoustic couplers were designed for TDD use and ran at about 50 baud.
I can tell you that 'way back when, a couple of us tried to see how
fast a pair of old Anderson Jacobsen modems would run and we didn't
get past 450 baud--and much less than that when calling Los Gatos
(Bay area old-timers may remember this).
Again, other than for the level conversion to RS-232C levels at the
output, these were strictly analog devices.
"Modern" 56K modems rely on the the digital 64Kbit nature of phone
lines and require a digital interface to the line at the far end.
The 56K arises out of the simple fact that, while the telco uses a
64K bit rate, some of the bits are reserved by the telco for
signalling and control. You can't do 56K over a strictly analog
phone line--if digital capabilities aren't sensed, the protocol
degrades to V.32/34/34 bis.
I believe that 8 carriers are used with 128 levels per carrier,
although it's been awhile since I read the standard.
Cheers,
Chuck
7:40 p.m.
You can't do 56K over a strictly analog phone
line--
Um, POTS phone lines *are* strictly analog, from the modem to the CO.
I wouldn't want to bet that 56K is impossible even with POTS copper at
each end, with suitably smart probing of the signal path and adaption
to its observed characteristics. (Which is not to say that modern 56K
modems can do that.)
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9:31 p.m.
On 25 Mar 2007 at 19:40, der Mouse wrote:
As I understand it, that's not the way 56K works--you can hook two
56K modems back-to-back on your workbench and they won't do 56K. As
I understand it, on end has to interface to a digital phone line.
One A/D conversion allowed. Hence, the line from you to your ISP is
not "striclty" analog.
See:
http://www.56k.com/basics/basics.shtml
Cheers,
Chuck
You can't
do 56K over a strictly analog phone line--
Um, POTS phone lines *are* strictly analog, from the modem to the CO.
I wouldn't want to bet that 56K is impossible even with POTS copper at
each end, with suitably smart probing of the signal path and adaption
to its observed characteristics. (Which is not to say that modern 56K
modems can do that.) 
11:22 p.m.
At 07:31 PM 3/25/2007, Chuck Guzis wrote:
As I understand it, that's not the way 56K
works--you can hook two
56K modems back-to-back on your workbench and they won't do 56K. As
I understand it, on end has to interface to a digital phone line.
One A/D conversion allowed. Hence, the line from you to your ISP is
not "striclty" analog.
It says you can't have more than one. It didn't say
that one was necessary. That web page dates from late 1996
or early 1997, too.
http://www.56k.com/basics/3things.shtml
- John
10:30 p.m.
Chuck Guzis wrote:
>As I understand it, that's not the way 56K
works--you can hook two
>56K modems back-to-back on your workbench and they won't do 56K. As
>I understand it, on end has to interface to a digital phone line.
>One A/D conversion allowed. Hence, the line from you to your ISP is
>not "striclty" analog.
To which John replied...
It says you can't have more than one. It
didn't say
that one was necessary. That web page dates from late 1996
or early 1997, too.
Chuck is absolutely correct. One end MUST be a digital connection. For 56K,
on the ISP end, the line must be ISDN, PRI, etc. Regular analog POTS line
won't work on the ISP end. In ADDITION, there can't be more than one analog
to digital transition anywhere in the circuit from source to destination.
Bridge clips will also invalidate 56K connections on POTS lines.
Jay
11:10 p.m.
On 25 Mar 2007 at 21:30, Jay West wrote:
One end MUST be a digital connection. For 56K,
on the ISP end, the line must be ISDN, PRI, etc. Regular analog POTS line
won't work on the ISP end. In ADDITION, there can't be more than one analog
to digital transition anywhere in the circuit from source to destination.
Bridge clips will also invalidate 56K connections on POTS lines.
Apropos to the vintage aspect...
Telcos used to (maybe prior to 1990) offer copper-to-copper
connections for things like fire and burglar alarms. Some
enterprising souls were making a go of using these connections
instead of ISDN (probably over a short haul, it might work). I was
surprised at how quickly all of the RBOCs got rid of the all-copper
arrangement.
Cheers,
Chuck
26 Mar
26 Mar
6:16 a.m.
"Chuck Guzis" wrote:
Telcos used to (maybe prior to 1990) offer copper-to-copper
connections for things like fire and burglar alarms. Some
enterprising souls were making a go of using these connections
instead of ISDN (probably over a short haul, it might work). I was
surprised at how quickly all of the RBOCs got rid of the all-copper
arrangement.
They still offer this (Verizon, anyway). It's tariffed.
I do this now with 2 SDSL modems I bought for $25 on ebay.
The copper is $40/month and I get about 1mbit/second between my house
and office. I run off-site backups and I'm typing this using VNC
over the same link.
-brad
6:50 a.m.
On Mar 25, 2007, at 11:10 PM, Chuck Guzis wrote:
Apropos to the vintage aspect...
Telcos used to (maybe prior to 1990) offer copper-to-copper
connections for things like fire and burglar alarms. Some
enterprising souls were making a go of using these connections
instead of ISDN (probably over a short haul, it might work). I was
surprised at how quickly all of the RBOCs got rid of the all-copper
arrangement.
That's not so apropos to the vintage aspect, actually. I managed
a DSL plant as one of my company's "side projects" using telco dry
copper until I changed jobs four months ago. We had maybe thirty
lines on that setup. Many (most?) telcos will still happily sell dry
copper circuits even today.
Prior to that, my connectivity used the very same dry copper setup
for two years, until I moved from Maryland to Florida five years
ago. My employer at the time (which had about 80Mbps of external
bandwidth) was right next door to the telco C.O., and both were about
two blocks from me. I had 2.5Mbps bidirectional, and it was rock
solid...that was verrah nice.
Though I was the closest, about a dozen other people in town had a
similar setup; their speeds ranged from about 2Mbps down to about
512Kbps.
In my case, it was "instead of a T1", not "instead of ISDN", as
I'd sooner exercise the bandwidth of a station wagon full of magtapes
before actually depending on ISDN for anything serious.
-Dave
--
Dave McGuire
Port Charlotte, FL
4:50 p.m.
Rumor has it that Dave McGuire may have mentioned these words:
In my case, it was "instead of a T1", not
"instead of ISDN", as
I'd sooner exercise the bandwidth of a station wagon full of magtapes
before actually depending on ISDN for anything serious.
Define "serious."
When I last moved (5 years ago), I went from a place that then (and now)
had at best 48K modem connections; usually worse, and nothing faster... and
I was paying over $80/mo. for two phone lines. The wife had an in-home
daycare, and I couldn't fight her job for line rights to do my job.
ISDN was available at the new place (and was the fastest available at the
time) and just having a line that _always_ gave me 64K up & down (and could
pair them for 128K as necessary) made multiple Telnet/SSH sessions a
breeze... (and Diablo II was quite happy as well)... at $62/month.
Gosh, pay less, go faster, rock stable. No brainer! ;-)
Then for a little while, I had a 1 Mbps bidirectional wireless, the upload
speed was nice, but it's ping/response times could get slower than my ISDN,
and for SSH it actually wasn't as nice... but the price was right... free. ;-)
My now current (overpriced) connection [[required by my now current
employer]] which is 3Mbps Charter line plays WoW allright I guess, but the
upload speed sucks, as I do a lot of uploading for work.
I still have the ISDN (but right now it's just a phone line, as I don't
think anyone supports it in my town any longer :-( ) and if I had to go
back to it, I wouldn't have a problem in the slightest. It was dependable
to the extreme, albeit slow(ish) by today's standards (read: YouTube).
Laterz,
Roger "Merch" Merchberger
--
Roger "Merch" Merchberger -- SysAdmin, Iceberg Computers
zmerch at 30below.com
Hi! I am a .signature virus. Copy me into your .signature to join in!
27 Mar
27 Mar
4:05 a.m.
On 26/03/07, Roger Merchberger <zmerch-cctalk at 30below.com> wrote:
When I last moved (5 years ago), I went from a place that then (and now)
had at best 48K modem connections; usually worse, and nothing faster... and
[..]
ISDN was available at the new place (and was the
fastest available at the
time) and just having a line that _always_ gave me 64K up & down (and could
pair them for 128K as necessary) made multiple Telnet/SSH sessions a
breeze... (and Diablo II was quite happy as well)... at $62/month.
~10 years ago we used to set up internet access in very small)
businesses with little d-link analogue routers - 33.6Kbps downstream,
if they were lucky. At least "flat rate" charging on the phone calls
had just come in - a few customers had had very nasty surprises with
chargeable calls when /quarterly/ phone bills had come in and they
discovered employees leaving PCs running radio streams on all day., or
backgrounded web pages with refresing adverts, or just windows.. and
the connection time was abysmal going from idle to connected, on
analogue, especially when you were at the other side of the building
and couldn't hear the modem dialing..
When we started putting in ISDN, it was a world of difference.
Reliable, quick (64 or 128K), instant connections.. even good enough
to leave online and support SMTP email... Only slowed down when ~30
people decided to browse the net at lunchtime all at once..
BT have pretty much discontinued ISDN2 over here now. With most of
the population able to have ADSL in some form, they see little demand.
What ADSL doesn't do well, of course, is point to point links, being
aimed squarely at internet access! We had more than one customer use
ISDN directly between sites for linking their networks. Migrating
these to VPNs via the internet was certanly possible, but whilst it
increased the bandwidth, they were certainly not as responsive or as
reliable.
Rob
26 Mar
26 Mar
11:37 a.m.
Chuck Guzis wrote:
On 25 Mar 2007 at 21:30, Jay West wrote:
A company I used to work for had numerous direct-wire
connections of this sort.
You could get either 2-wire or 4-wire (2-pair) connections. We
always used the 4-wire (2-pair) option. You could then attach a
"short haul modem" on each end to get a serial line that would
work up to 9600 baud, depending on the distance.
One of the biggest problem we had, was the phone company
inserting "transformers" in the leased lines. They would do
this on their own initiative. This would cause the modems to
quit working. After getting them to remove the transformers,
we would have to re-determine the pairs.
One end MUST be a digital connection. For 56K,
on the ISP end, the line must be ISDN, PRI, etc. Regular analog POTS line
won't work on the ISP end. In ADDITION, there can't be more than one analog
to digital transition anywhere in the circuit from source to destination.
Bridge clips will also invalidate 56K connections on POTS lines.
Apropos to the vintage aspect...
Telcos used to (maybe prior to 1990) offer copper-to-copper
connections for things like fire and burglar alarms. Some
enterprising souls were making a go of using these connections
instead of ISDN (probably over a short haul, it might work). I was
surprised at how quickly all of the RBOCs got rid of the all-copper
arrangement.
6:12 a.m.
"Jay West" wrote:
Chuck Guzis wrote:
>>As I understand it, that's not the way 56K works--you can hook two
>>56K modems back-to-back on your workbench and they won't do 56K. As
>>I understand it, on end has to interface to a digital phone line.
>>One A/D conversion allowed. Hence, the line from you to your ISP is
>>not "striclty" analog.
Last time I did this (we make a small box with 2000 modem modems in it),
the modems were fed from a T1/T3 with digital data.
The subscriber's data came from the CO in digital form and hit our
modems as digital data. No doubt it started as analog on their loop and
immediately got digitized (7 bits) and stuffed into a T1 where it stayed
digital until it hit us.
The modem code was highly tuned for this configuration.
Bridge clips will also invalidate 56K connections on
POTS lines.
ps: I'm not sure I believe this... if it says "may effect and possible
degrade" I'd sign up.
-brad
25 Mar
25 Mar
10:14 p.m.
der Mouse wrote:
I wouldn't want to bet that 56K is impossible even
with POTS copper at
each end, with suitably smart probing of the signal path and adaption
to its observed characteristics. (Which is not to say that modern 56K
modems can do that.)
That is the other reasion I stay on dial up.
I don't think the phone lines here can handle high-speed
to my apartment.
/~\ The ASCII der Mouse
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.
26 Mar
26 Mar
7:02 a.m.
On Mar 25, 2007, at 7:40 PM, der Mouse wrote:
Not necessarily. Here, for example, it's analog from my house to
the SLC-96 in the CEV vault up the street. From there to the C.O.
it's digital. This is very, very common.
-Dave
--
Dave McGuire
Port Charlotte, FL
You can't
do 56K over a strictly analog phone line--
Um, POTS phone lines *are* strictly analog, from the modem to the CO.
12:44 p.m.
Dave McGuire wrote:
On Mar 25, 2007, at 7:40 PM, der Mouse wrote:
Not necessarily. Here, for example, it's analog from my house to
the SLC-96 in the CEV vault up the street. From there to the C.O.
it's digital. This is very, very common.
if there's a SLIC in place, what happens if you ask for an alarm circuit
(i.e. "dry copper")? I've always wondered...
-brad
You
can't do 56K over a strictly analog phone line--
Um, POTS phone lines *are* strictly analog, from the modem to the CO. 
25 Mar
25 Mar
12:20 p.m.
We will get several comments after this post...
I hope y'all will correct me.
The acoustic coupler on the old teletype was 1200 baud, 10 characters per
second
That to me implies 120 signal line changes for each character. Can you
please explain what the heck was going on :-)
More seriously, most old TTY modems _were_ 10 characters per second, but
were 110 baud. 11 signal line changes per character -- start bit, 8 data
bits (or 7 data + parity), 2 stop bits.
-tony
27 Mar
27 Mar
7:57 a.m.
On Sun, 2007-03-25 at 17:20 +0100, Tony Duell wrote:
ASR33 with a big Type-R sticker on the side ;-)
Gordon
We will get several comments after this post...
I hope y'all will correct me.
The acoustic coupler on the old teletype was 1200 baud, 10 characters per
second
That to me implies 120 signal line changes for each character. Can you
please explain what the heck was going on :-)
6277
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