Telecommunications Tech Blog November 2014
What's Sidetone, and Why Do You Need it?
Sidetone is when you hear a little bit of your own voice in your ear as you are
talking on a traditional phone / handset. For most of us it makes it more
comfortable to talk on the phone.
Phones have had sidetone for over a
hundred years. It was originally put there at a particular volume (fairly low)
so the caller would get an idea if they were talking too loud, creating
distortion on the old transmitters.
Phones universally had sidetone until
smaller cell phones became popular. On those phones they removed the sidetone to
prevent feedback (squeal) that you get when the mic is too close to the speaker.
There is no sidetone on speakerphones.
When VoIP phones came out some
manufacturers decided to remove sidetone as an easy way to prevent echo, rather
than designing the telephone properly. While a phone is usable without sidetone
for normal conversation, it's not much fun to use. One of the benefits of
sidetone is that you know if you've been cut-off from the call. A lot of us have
found ourselves talking for a while on our cell phone, only to realize that
there was nobody on the other end because the call was dropped.
When key telephones and single line
telephones used analog voice, businesses were able to record or monitor (called
"service observing") from the analog voice pair going to the phone. Many older
key telephones used an analog pair for the talk path for voice, and a second
digital pair for signaling (line selection, lights, ringing and hold). A voice
activated (vox) recorder connected to the analog voice pair would record all of
the conversations on that phone, time stamp them, and let you search for the
When phones came out with digitized
voice (as well as signaling), and then VoIP phones became popular, the only
place they were analog was at the handset (since we all still have analog ears
Actually, you can record VoIP calls
by copying the VoIP voice packets (RTP packets) as they go into your router via
Ethernet with a special VoIP logger. The logger assembles the packets from the
conversation and saves the conversation as an audio file (like an mp3).
Many VoIP PBXs have a call recording
feature built-in, and some digital phone systems like ESI have had an internal
call recording function for many years.
You can play with it yourself if you
put Wireshark on a laptop and setup a mirrored port on a managed Ethernet switch
to monitor all the traffic on the port that's going to the router. Save all the
packets to a file in Wireshark, then click on Wireshark's VoIP button and tell
it to play back a call from whatever IP address you'd like. You'll hear the
whole call - both sides of it. Note that this won't work if you're using a
secure VoIP format that encrypts voice packets.
sounds complicated, there's an easier way!
For digital phone system phones as
well as VoIP phones, it's often easier and cheaper to record from the only place
it's analog... off the receiver wires of the handset jack.
The handset jack and a modular
headset jack has another a real benefit as opposed to recording from an analog
station pair / POTS line. When you record from an analog pair (that works with a
standard analog home phone) the local person is often recorded at a much louder
level than the person at the other end. Some voice loggers have an automatic
gain control that tries to fix that problem.
When you record from the handset jack
(the receiver wires) on a normal phone with sidetone, the levels of both parties
is about perfect. The person on the phone locally is heard at the lower sidetone
volume level, and the person at the far end is recorded at normal level. It just
plain works great for recording or service observation.
Keep in mind that a modular
headset jack is often wired differently from a modular handset
jack, mainly so you can plug a headset directly into the phone without the need
for an amplifier in-front of the headset.
On business phones with a 2.5mm audio
plug for the headset (used on older cell phones), there's a chance that there
will be no sidetone on the headset jack. And, recording from a 2.5mm headset
jack is difficult because the 2.5mm jack only has three wires, not four. With
the common ground for transmit and receive there may be hum or noise if you try
to record from that jack.
Most real voice loggers made for
recording analog phone lines or analog stations are transformer isolated, and
the inexpensive Handset Y Adapter (above) using a spare pair to the
recorder from the Handset Y Adapter will work fine.
For tape recorders, PCs and anything
not made to be connected to a real phone line, you'll need to get a Deluxe
Handset Recording Adapter which transformer isolates the phone from the
recording device so you don't hear hum and noise:
You need to isolate the electronic
telephone (which has a reference to ground through the phone's or system's AC
power) and the recording device with an isolation transformer. Otherwise you get
a ground loop or foreign voltage... and the resulting hum and noise.
There are lots of voice loggers on
the market these days. If you need to record all the stations to a single logger
the cost of mutli-channel loggers has come down in the last few years, but they
aren't cheap. When recording from the handset, you send the receiver audio down
a spare pair to the phone room, and cross connect that pair into a channel on
the voice logger (usually placed in the phone room).
For recording a single phone right at
the desk onto the Windows workstation, we recommend an inexpensive Logging
The program is vox operated (voice
activated) so you just leave it running in the tray of the PC and it records all
the calls coming into the mic input on the PC from the Deluxe Handset
Recording Adapter. We've used it for about a decade, and it works perfectly.
Totally automatic. You don't have to start or stop recording manually, although
you can if you want. It has its own compression algorithm that saves a lot of
disk space and the call is perfectly understandable (calls can be exported to
wav files after they are logged).
Sometimes you need multiple people to hear a conversation in the same room or
another room as someone on a handset (and you don't want to use a speakerphone
because the person on the other end will be able to tell). The Orator III
one-way amplifier / speaker will plug into an analog line or station port, as
well as to the handset on a digital or VoIP phone using a Handset Kit:
As long as you hear sidetone in the
handset or headset jack, you'll be able to hear both sides of the
conversation when recording or monitoring.
If you don't hear a little of your
own voice in your ear, you don't have sidetone and when you record or monitor
you'll only get the party at the far end, not the person on the local phone.