Audio Codecs


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Sending audio from remote locations has never been easier

At one time, all remote broadcasts involved a long wait from the phone company while an expensive equalized circuit was engineered and installed. The alternative was a remote pickup (RPU) transmitter. While the need for a long lead time was avoided, obtaining a clear path to the receiver was not always possible. In some areas, finding an available frequency is nearly impossible. For many stations, these two options are still the normal way of doing things. In the past few years a new method has been added to the station's bag of tricks. The codec (shortened from encoder/decoder) has opened the door to a new level of quality for remote broadcasts, eliminating the dependence on radio links and special-order telephone circuits.

Whether you are a seasoned professional or new to broadcasting, the choice of which codec to purchase should not be a snap decision. Simply picking a specific unit because the station across town bought one might lead to more questions than answers. Before you pick out a unit, you should be fully aware of the station's needs. This is a good item to discuss with your fellow department heads. You will likely find that sales and programming have different needs for remote equipment. Most stations do several types of remotes including night clubs, car dealers, local fairs and sporting events. These all present different challenges and different requirements not only for the equipment, but also for the person engineering the remote.

It is important to look at how you will use the equipment, but it is equally important to look at the rest of your facility. How will the equipment integrate into your existing remote equipment package? Will it work with your intercom system, and will there be any problems with too much data reduction in your audio path to the transmitter? With several popular coding algorithms, care should be taken not to intermix too many different algorithms along the audio path. Stacking too many algorithms can cause degradation in the overall audio quality and can cause audible anomalies to appear in the on-air sound. On top of the decision of how to code the audio, the method of delivering the coded audio introduces just as many questions. ISDN and POTS (Plain Old Telephone Service) lines provide different levels of bandwidth to transport different methods of coding.

Side effects One artifact of coding audio is time delay. All methods of coding introduce some finite amount of delay into the audio path. Some are on the order of a few milliseconds, while some coding methods introduce a delay long enough to make it difficult to monitor source audio on the return path. Delay times approaching 0.25 seconds and longer are possible. The station must provide a mix-minus so the local audio and studio backhaul can be monitored.

The first question you need to answer regards cost; not only cost of the equipment, but the cost of doing the remote. Look at the number of remotes per week - how much does the sales department charge? Are they only doing one or two live breaks per remote, or are they doing a full-music show from a night club? Does programming want or need to have full stereo audio, along with a closed-circuit station-monitor feed with IFB and a data port from the station LAN, available at the remote site? Does the promotions department need station program audio available to feed a PA system? Finally, do you have the time or manpower to go to every remote, or do you need to use non-technical personnel to operate the equipment? These answers will help determine what method of program delivery best suits the station's needs.

Once the station's needs are assessed, start looking at equipment. Usually, basic remotes will be just fine with lower bandwidth systems, while more complex remotes will require considerably more bandwidth. With more bandwidth comes more complex equipment. The more complex the equipment and the remote, the more engineering will likely need to be present to handle setup, teardown and equipment operation during the remote.

For basic remotes, POTS codecs offer the most overall flexibility in the smallest package and are usually able to transmit bi-directional mono audio up to 15kHz with a connection rate of approximately 33.6kb/s. The ability to use any standard telephone line allows almost any location in the world to be remote-broadcast ready. A slower connection rate will provide less bandwidth in the audio channel, but even at a slow modem connection, 8kHz of audio bandwidth is more than acceptable for voice-grade audio.

Getting connected POTS codec setup is typically simple because the number of setable parameters is minimal. POTS codecs operate similar to standard computer modems, going through a short dialing and handshaking period before passing audio. Many units include a small two- or three-channel mixer that provides control over the input levels as well as headphone outputs and a local/return monitor mix. Most well-equipped POTS codecs can stand alone without any additional support equipment.

Since these units operate on a standard telephone line, even the most inexperienced operator can be taught how to use it in a short time. It is often a greater challenge to convey the simplicity of the operation than it is to train the operator on its use.

Higher bandwidth For more involved remotes, ISDN provides a full 20kHz stereo bandwidth and bi-directional audio paths. With the proliferation of ISDN around the world, more complex remotes can be done from more places than ever before. What once required coordinated satellite channels and a POTS line for cueing and IFB can now be done with a single ISDN line. Full-stereo program audio can be shipped to the studio at the same time that mono program mix-minus and cue audio are sent back to the remote site for the talent.

The one difficulty with ISDN over POTS lines is the amount of expertise involved to make it work. Orders for ISDN lines must be placed and configured to work correctly with the chosen equipment; service profile identifier (SPID) numbers must be entered correctly into the equipment; and protocols must be matched between the studio and remote site for the link to work properly. One incorrectly entered digit can cause the ISDN interface to communicate improperly with the phone company's central office.

A benefit of ISDN is the option to use different algorithms depending on the type of program material. Mono voice can be carried using the G.722 algorithm with a 6ms delay while still providing more than acceptable audio quality. For music remotes, MPEG-1 LayerII or LayerIII will provide better audio quality but with a longer delay. There are variations of bit rate, sampling frequency and mono or stereo settings that affect the amount of time delay. See Table 1 for a comparison of the timing differences.

Because ISDN remotes tend to be more complex, an on-site engineer is usually necessary during the remote. The high-quality audio and imbedded signalling functions make it possible to move the entire studio outside the broadcast facility for short periods of time. Some ISDN codecs have built-in mixers and internal headphone amplifiers. Codecs without these features require additional equipment. More recent ISDN codec introductions have been designed for fast and simple setup and operation.

Both styles of codec offer an auxiliary data path. The data rate is usually around 2.4kb/s to 4.8kb/s. Some units also offer virtual switch connections. These functions can be used to remotely start events such as a CD player or control equipment through the RS-232 port. The data path may be used for basic text messaging between the remote site and the studio producer, or it may be used to control a piece of equipment such as an on-air telephone system.

Another growing use of ISDN is for backup studio-to-transmitter links. The almost instantaneous connect time after dialing and the full 20kHz bandwidth stereo audio can provide a full backup to traditional STLs and inter-city relay links.

Multiple personalities The basic operation of POTS and ISDN codecs is the same. The transmission media (the phone lines) have very different characteristics, but once a signal is digitized and compressed, it is passed on to the modem. Some codecs have multiple personalities that provide additional flexibility to the user. POTS and ISDN modems can be fitted into a single unit. More recently, wireless applications like PCS and GSM have proved to be viable methods as well. Stations faced with a variety of locations may take advantage of this feature.

In some cases, the extreme data reduction of a single ISDN line is not acceptable. Classical music broadcasts are typically less forgiving with aggressive encoding schemes. A violin can be made to sound like something completely different with the wrong mix encoding algorithms. In cases where higher audio quality is needed, the capacity of a single ISDN line is quickly exceeded. Some codecs allow multiple ISDN lines to be bound together to act as a single, larger pipeline. Instead of only 128kb/s capacity, 256kb/s or 384kb/s can be used with two or three ISDN lines.

Binding multiple lines for increased capacity is not something that most stations will routinely attempt. However, when the situation calls for the increased bandwidth, multiple ISDN lines may be a better alternative than fractional T-1.

When you make your codec purchase decision, keep in mind that at least one of the units will be going on the road. It may be in the hands of both technical and non-technical personnel. All codecs are built to withstand normal use, but remote equipment does not enjoy the same delicate lifestyle that rack-mounted studio equipment does. The attractive molded plastic case will not be very attractive once it has been dropped. Equipment designed for rack mounting may not survive the same bumps and jars once it is on the road. Be sure to include the appropriate carrying case or road hardware to protect your investment.




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