The Basics
ISDN, and its predecessor Switched56, are technologies for sending high-quality audio on dial-up telephone lines. This is not to say that any old phone line can be used for the purpose, it requires a specially configured telephone line as well as a small pile of equipment at each end.
The big advantages are immediate availability, bi-directional capability and low cost. The technology now exists for dial-up stereo remotes.
ISDN has become popular, too, with the Internet crowd, for connecting to Internet Service Providers to send data to and from the Internet.
If you think of an ISDN circuit as simply three pipelines: two "pipes" to carry data and one "pipe" to provide data coordination for the other two.
The Equipment
You'll need some source of audio, at the very least a microphone and probably a tape
recorder of some description. Then you need to be able to control the level, lets add a
mixer or Loudmouth.
(The Loudmouth is an NPR-designed, Benchmark Systems-built audio switching device that
routes analog audio into and from line-level devices. It also contains a mic preamp that
can also be routed into connected devices, metering and switchable headphone monitoring).
Finally, you need to monitor the the outgoing and return audio (ISDN and Switched56 connections have the ability to send audio in both directions simultaneously), headphones are probably preferable since we have an open mic. The audio level to and from the ISDN equipment is a broadcast line level, +4dBu.
There are four components to an ISDN audio package. There is an Audio Encoder which converts audio to data. The Audio Decoder does exactly the opposite, converting data to audio. The Terminal Adapter which connects the data (encoder & decoder) with the 4-wire ISDN interface and allows you to control various aspects of the call (like where it goes). Finally there is the NT-1 which converts the 4-wire interface from the terminal adapter to the 2-wire standard used by the phone company.
You will probably never see four separate boxes. The various manufacturers have combined these components together in different combinations.
The Call
ISDN/Switched56 service can be broken into two parts; the Local Access line at each end and the Long Distance carrier. You will notice that this is very much like the standard telephone network because that is precisely the case. This is merely a telephone connection that is guaranteed a 64K bits/second or 56k bits/second path all the way.
Audio Quality
The resulting quality of the audio depends on four major factors:
- Algorithm
- Data Rate
- Sample Rate
- Mode
The Algorithm is merely an agreed upon way to utilize the bits. Since we have 64,000 bits per second, we could allocate a such-and-such number of bits to describe the audio waveforms, another number of bits allocated for error correction, and so on. Some algorithms are international standards, such as ISO MPEG Layer 2, or G.722, that any manufacturer can use in their ISDN codecs. Other algorithms, such as the MUSICAM protocol, belongs to MUSICAM USA, and other manufacturers must buy a license from MUSICAM USA in order to support this algorithm. Competition to make the best sound quality while using the fewest bits is intense.
Data Rate is the bitrate of our ISDN or Switched56 line. 64K means 64,000 bits per second. When two 64K channels are used together as one, the resulting data rate is 128,000 bits per second. Adding channels together is called bonding, and is useful when you want to get the very best quality stereo sound possible.
Sample Rate is the number of times per second the audio waveform is analyzed, or sampled. Interestingly, higher sample rates do not always return better quality audio, unless the bit rate is increased also.
Mode refers to the audio configuration, whether stereo, monaural, joint stereo, or dual.
In general, here is how the systems line up from highest to lowest in Monaural:.
| Algorithm | Data Rate |
Sample Rate |
Comments |
|---|---|---|---|
| Layer 2 | 128K | 48K | As used for "mono" channels in the Public Radio Satellite System |
| Layer 3 | 64K | 32K | The original Zephyr coding scheme |
| Layer 3 | 64K | 48K | Poorer error correction than 32K sampling |
| Layer 2 | 64K | 24K | MUSICAM TM Protocol. 10.5 KHz audio bandwidth with good error correction |
| Layer 2 | 64K | 48K | 8.5 KHz audio bandwidth |
| G.722 | 64K | 16K | 7 KHz audio bandwidth,with little delay. Almost all codec support G.722, as it was one of the first algorithms developed. |
The big advantages are immediate availability, bi-directional capability and low cost. The technology now exists for dial-up stereo remotes.
| Algorithm | Data Rate |
Sample Rate |
Comments |
|---|---|---|---|
| AC2 | 384K | 48K | Gotta be the best |
| Layer 2 | 384K | 48K | Very good transcoding |
| AC2 | 256K | 48K | The standard for DolbyFax Stereo |
| Layer 2 | 256K | 48K | Same as the Public Radio Satellite System |
| Layer 3 | 128K | 32K | Best possible setup at 128K for Broadcast |
| Layer 3 | 128K | 48K | Better frequency response but poorer error correction |
| Layer 2 | 128K | 32K | OK response but poor separation |
| Layer 2 | 128K | 48K | Last fallback for Stereo |
MPEG Layer 2 and 3 both offer Stereo and Joint Stereo. The rule of thumb is that channel separation will be better with Stereo while frequency response will be better with Joint Stereo. Joint stereo should only be used at 128K.
Locations
There are several sources to find locations that have ISDN or Switched56:
- Within EUonline, you can browse
our ISDN database, showing station/company names,city,state/country,bitrate and codec
type.
Our database has public radio stations and commercial operations as well.
Please note that our database does not have contact telephone numbers, and that you need a JAVA-enabled browser to view the list.
- For U.S. domestic information, see Al Rieland's Switched Digital Audio Directory for Public Radio
- Jay Rose's Audiobahn, formerly known as ZephyrSpace.
- For International locations, see Ian Briton's Broadcast ISDN User Guide & Directory.
NPR Washington
NPR Audio Engineering maintains a variety of codecs to ensure compatibility with most other locations. There are, however, a number of coding schemes in use in Europe that we do not now, nor do we have plans to, support.
MC ISDN-1 (PHON 11) MUSICAM/G.722, 56/64KBit
MC ISDN-2 (PHON 12) MUSICAM/G.722, 56/64KBit
MC ISDN-3 (PHON 13) MUSICAM/G.722, 56/64KBit
MC ISDN-4 (PHON 14) MUSICAM/G.722, 56/64KBit
MC MUSICAM (PHON 15) MUSICAM Stereo
MC ZEPHYR (PHON 16) MPEG Level II/III/G.722
MC RE-661 (PHON 17) MPEG Level II
RC ISDN-81 (PHON 81) MUSICAM/G.722, 56/64KBit
RC ISDN-82 (PHON 82) MPEG Level II/III/G.722
The SPIDs for all NPR Washington ISDNs follow the patters 01XXXXXXX000 where XXXXXXX is the seven digit local ISDN number.
NPR's Satellite Technical Center has additional ISDN service.
