I really wish Australia had Radio Shack.
August 24, 2011 4:48 AM   Subscribe

I need a basic landline phone recording lead like this. I might not have enough time to order one online. Can I make one myself? How would I do that? (Bonus question: Where can I buy one in Sydney?)

I need to record some conversations from my home phone. (It's completely above-board - they're planned, consensual interviews for an academic paper, and everyone will know they are being recorded.)

I already have an audio recorder. I just need a lead to connect it to my landline phone. I haven't been able to find one locally and I might not have enough time to order one online. They look pretty straightforward - am I kidding myself to think that I could make one in an evening with a phone cord, an audio lead and some wire strippers? Could those of you familiar with electronics/audio give me some advice on how to do that?

Alternatively, can you tell me where I could buy a ready-made lead in Sydney? I'm pretty sure such equipment is legal in NSW, but given that it's a dual-consent state, the legal uses for it among the general public are pretty limited - perhaps enough to make it a non-viable market. I've tried the obvious places like Dick Smith, but maybe there's a secret spy shop somewhere I'm not aware of?

(I'm aware that there are other options - Skype, my mobile phone, special phone recorders etc. For various reasons, recording directly from my landline would be the most convenient in this particular situation.)
posted by embrangled to Technology (19 answers total)
That lead appears to plug into the telephone jack which means that it has some circuitry to terminate the SLIC (subscriber loop interface circuit) sitting in the exchange down the street. It's not something you can quickly solder together.

Your best bet is to get your hands on a cheapie cordless phone with a 3.5mm headphone jack on the side and plug into that. You plug the phone into your landline jack and your recording equipment to the phone and you'll be set-up like Rose Mary Woods.
posted by three blind mice at 5:18 AM on August 24, 2011

Response by poster: Hmm, I feared it might be more complicated than it looked. Does it change matters at all that my phone line is actually a VOIP line? (By this I mean that I have a 'naked' ADSL connection and a VOIP line as part of that - I plug my ordinary phone into the phone port on my router and my router goes into the phone jack in the wall).
posted by embrangled at 5:23 AM on August 24, 2011

Response by poster: Also, forgive me if this is a stupid question, but wouldn't the circuitry inside my phone already be terminating the SLIC? Isn't the lead just 'listening in' on a connection that has already been established? My understanding is that it's meant to be used with a dual adaptor, either between the phone and its headset, or between the phone and the wall jack (or in my case the, router).
posted by embrangled at 5:32 AM on August 24, 2011

Your VoIP router terminates a IP-based connection and translates the received audio signals into a POTS (plain old telephone system) circuit. Your router provides the SLIC interface to your POTS phone which is why you can plug any old-fashioned telephone into your fancy VoIP router.

Yes, absolutely, the circuitry inside your phone terminates the circuit, but your problem is getting a tap onto the audio signal. My suggestion to use a cordless phone with a 3.5mm headphone jack gives you a point to tap the audio signal with good quality.
posted by three blind mice at 5:42 AM on August 24, 2011

Response by poster: Okay, that makes sense. I'd prefer not to use a cordless phone. I bought and returned three different models before giving up and buying my current old-style phone. Despite claiming to transmit on different frequencies to my wireless router, they all suffered interference that made them unusable. Also, the call quality isn't all that great for recording purposes (I know this because I've worked as a radio journalist and calls from cordless phones are never as clear).

Can you explain a little clearly more why a lead like this is something I couldn't make myself?
posted by embrangled at 5:58 AM on August 24, 2011

I only suggested a cordless phone because I know some models come with a 3.5mm headphone jack. I would be surprised if a DECT phone suffered from interference. The digital error correction is pretty robust.

Can you explain a little clearly more why a lead like this is something I couldn't make myself?

Because you have to terminate a circuit that looks like this.

There are some chips available that'll do the job, but you'll have some external circuits and wiring to make it work.

Another thing to do would be to build an inductive telephone tap which you mount on the back of your phone's handset and picks up the magnetic variations of the speaker coil, but this seems no less involved.
posted by three blind mice at 6:25 AM on August 24, 2011

Response by poster: Sorry to threadsit, but I'm doing my best to understand - didn't we establish that my phone and/or router terminate the circuit themselves? Why would the lead also need to terminate the circuit? The one I linked to doesn't seem to have room for circuitry of any kind - it's literally just an audio jack stuck on the end of a phone cord. Are you saying there's a chip inside it somewhere?
posted by embrangled at 6:32 AM on August 24, 2011

Apologies if this is very definitely not an answer, but there's always the possibility of using Skype (or another computer-based VoIP solution), at which point recording the audio becomes elementary. You can use Skype to call landlines, or have a landline number forward through to Skype.
posted by Magnakai at 6:41 AM on August 24, 2011

When I had to record phone interviews for a similar reason in college, I just used my answering machine. Given, this was 15 years ago and it was the type of machine that used mini cassettes, but there was an option on it to easily record a phone call. I'm sure modern answering machines, even the ones without cassettes, are likely to have a similar feature. Maybe a visit to a department store is in order if you don't already have one.
posted by cuddles.mcsnuggy at 6:52 AM on August 24, 2011

Failing that, just do the interview on speakerphone and record using any other recording device.
posted by cuddles.mcsnuggy at 6:53 AM on August 24, 2011

Best answer: Are you saying there's a chip inside it somewhere?

Yes. Probably embedded in the plastic plug that goes into the phone jack. That's why I say it's more complicated than simply twisting some wires together. This app note from Intersil gives you an idea of what the circuit looks like.

didn't we establish that my phone and/or router terminate the circuit themselves?

Yes, but the problem remains how to tap off the audio signal? If you want to bypass the phone, you have to use something else to terminate the SLIC which is what the lead you linked to appears to do.

In any case, since you do not need to hide this, why not just tap directly off the speaker in your telephone? On an old phone, I'd unscrew the earpiece, the speaker would just fall out held in place by two wires carrying the audio signal. Attach each wire from a 3.5mm adapter, plug it into a high impedance recorder and you have it.
posted by three blind mice at 7:33 AM on August 24, 2011

Best answer: OK… ex-Telstra switching tech here.

First up, you're not easily going to be able to buy one of those in Australia - customer equipment still needs to be ACMA approved, and there's no way in hell something like that would be approved for the purpose of recording calls. Yes, even equipment connected to a VOIP interface needs to be approved (though that's honoured more in theory than practice…)

Second: the easiest way would be to get a cordless with a headphone jack, and an adaptor cable from Jaycar or DSE (if you can find a DSE that hasn't been totally Woolworth'd).

Third: Yes, use your existing phone to handle looping / terminating the line.

Fourth: attaching stuff directly to the phone line / an old phone is iffy, as there can be up to 50V DC / 90+V AC there. I'd suggest not doing that unless you know something about electronics, in which case you would probably be able to make a reasonable guess at a circuit & not need to ask the question here.

Fifth: If you want to go that route, I'll have a think about the best way to describe the circuit here. Won't be until the morning, though.

Sixth: OzSpy in Brisbane sell them. I'm guessing similar shops elsewhere sell similar devices.
posted by Pinback at 7:46 AM on August 24, 2011 [1 favorite]

Response by poster: Fourth: attaching stuff directly to the phone line / an old phone is iffy, as there can be up to 50V DC / 90+V AC there. I'd suggest not doing that unless you know something about electronics, in which case you would probably be able to make a reasonable guess at a circuit & not need to ask the question here.

Well, I built a lot of battery-powered circuits as a kid, and I've successfully re-wired basic items like table lamps as an adult. My dad was an electrician, so I'm pretty serious about electrical safety. Is that the kind of knowledge I'd need to build a lead like this, or is it a lot more complicated than that?

I had sort of assumed I could chop the ends off a spare phone extension cable and an audio lead, strip the individual wires, find out from a knowledgeable source (hi, AskMe) which wires in the phone cable do what, solder or twist the right ones together and then insulate the heck out of everything.

If that's a dangerous or misguided thing to do, then of course I won't do it. If it sounds about right, though, I'd really appreciate some advice or learning resources. (And it's not just an idle whim - a radio journalist, knowing more about how analogue phones work can only be a good thing. There are still places where they're the best option for getting a story out. My much older colleagues tell stories about 'filing by alligator clips' but most of that old equipment is no longer in use).

Sixth: OzSpy in Brisbane sell them. I'm guessing similar shops elsewhere sell similar devices.

Brilliant! If making one turns out to be dangerous or impractical, I'll order from them.
posted by embrangled at 7:40 PM on August 24, 2011

Best answer: Fair enough. There's nothing particularly life-threatening to a phone line (you get used to being electrocuted by 16Hz 90+V A.C. eventually!), but it can damage equipment. Sorry, still haven't got around to drawing up a circuit, but the basic idea is this:

You want a DC-isolated attenuator, with something to clip/limit potential excessive AC spikes. A couple of capacitors, 1 in each leg, connecting to the line. The other end of them connects to a voltage divider, the exact ratio of which will depend on a few factors the signal level required by your input. Then, to cap it all off, a couple of back to back zeners to limit AC.

Trying my hand at ASCII art…
F    <--| |---+
r             |
o             R1
m             |
L             |     |    |
i             R2    Z1   Z2     To recorder input.
n             |     |    |
e    <--| |---+-----+----+----->
I'd suggest 0.1 ~ 0.5uF for the capacitors, R1 should be something like 10k, R2 something like 1k, and Z1 & Z2 should be zener diodes somewhere in the vicinity of 3V~5V. Z1 should be connected with the +ve end towards the top; Z2 with the positive end towards the bottom (or vice-versa - doesn't really matter as long as they're opposite to each other!). It's not particularly polarised; either input lead can connect to either leg of the phone line, and the same goes for the recorder end.

That'd be where I'd start anyway - if the level is too high, increase R1 (or decrease R2); if it's too low, decrease R1 (or increase R2).

IAAT, but IANYTechnician; if this breaks your phone line, VoIP ATA, recorder, or spine, you get to keep the pieces. 10c refundable deposit in S.A. only.

(FWIW, that's essentially the input circuit of the monitoring amps Telstra used to use.)
posted by Pinback at 8:49 PM on August 24, 2011

(Also: there's no way in hell that'd be approved by the ACMA for conection to a phone line; it'd need a suitable line isolating transformer, for one. Don't make them and sell them to anyone, and for Zod's sake don't tell the police, Feds, or ACMA investigators you got it from me!)
posted by Pinback at 8:57 PM on August 24, 2011

Engadget has a post showing how to build a Telephone Recording Circuit out of an old modem, complete with pictures. The circuit looks quite similar to the one Pinback posted above, but I thought the pictures and directions might be helpful to you.
posted by slicesoftree at 5:15 AM on August 25, 2011

Interesting… it's quite similar, the main difference being that they do use a line isolation transformer (the only bit of the modem they use).

Which I figure is largely pointless - it's the capacitors that do the D.C. isolation; the transformer is only there to give the 5kV (or whatever, I forget) isolation between sides required for type approval. Since a home-brew job is never going to get type approval, and capacitive isolation is (was?) good enough for individually-approved devices, it's largely moot.

If you want to go that way & don't have any modems to spare, Jaycar sells a couple of versions of phone line isolation transformers. You might go scrounging an old modem, or try my circuit after all, when you see the price ;-)
posted by Pinback at 3:56 PM on August 25, 2011

Response by poster: Thank you all so much for your advice. It's actually been great to learn more about how phones work, but it looks like the circuit is complex enough that I'm better off just ordering the lead online. I'm surprised by how much circuitry is apparently packed into such a device - the lead I linked to, especially, doesn't seem to have room for it all. Don't worry, Pinback, I have no intention of getting you or myself into trouble - now that I understand the complexity of what I was considering messing with, I think it's best I don't mess with it at all. I had assumed that the cable between the phone and the handset carried simple audio, which ought to be easy enough to tap - evidently that's not the case, so I'm glad I asked.

While I wait for the lead to turn up, I'll play around with hooking up some alligator clips to the speaker inputs in my phone handset. It certainly helps that my phone is one like this, so I have no excuse for not being familiar with its anatomy.
posted by embrangled at 4:57 AM on August 26, 2011

Response by poster: Update: I tapped the audio circuit of my phone and got a surprisingly good quality recording.

I bought a length of audio cable and wired a 3.5mm plug to one end and a set of alligator clips to the other. Then I took the back off my phone's handset and attached the clips to the contacts on the speaker. I taped the handset back together, put the plug into my audio recorder and voila! A recordable phone.

Thanks to those who took the time to explain what I hadn't really understood: that phone cords carry electrical signals, not pure audio, so trying to tap a phone through its cord is a tricky and complicated business. I'm glad I didn't try to go down that route. Tapping at the speaker was much, much simpler and gave a very good result.
posted by embrangled at 3:19 AM on September 29, 2011

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