Is it Time for a New Transmitter?
Going one more
Let's take this N+1 idea a little farther and see what is available in terms of broadband antennas. Shively makes the model 6832, which is broad-banded (VSWR 1.35:1 or less across the entire FM band) and will handle up to 2.5kW per bay. Another possibility for a low power N+1 application could be the Jampro JCPB. Its VSWR spec is 1.25:1 or better across the entire FM band, and it's also rated at 2.5kW per bay.
On the opposite end of the power scale for Harris Broadcast is the HPX, which is its high-power, vacuum-tube PA line of transmitters. According to Harris, the HPX system "provides the highest operating efficiency of any transmitter in its class" thanks to its 1/4-wave cavity and grounded cathode design. Its basic on-board controller provides critical life support, control, automatic exciter switching and fault diagnostic functions without reliance on a microprocessor. The enhanced transmitter control unit option combines the basic controller functionality with a microprocessor and 1/4 VGA touch-screen GUI for increased control and diagnostics, including TCP/IP connectivity, Web remote control/monitoring and SNMP support.
The HPX can also be used as a combined amplifier for HD Radio purposes. As an example, in conjunction with the Flexstar exciter, and using the real-time adaptive correction feature, the HPX-40 can make 31.5kW FM and digital at the -20dBc level, or 17kW at the -10dBc level.
For nearly my entire career I've been connecting a stereo generator output to an exciter input via a piece of coax, although there was that time, in the 1990s, where I experimented with running AES straight in to an exciter. The results were mixed - I was soon back to the coax runs. The Nautel Omnia Direct interface makes an AES connection from an Omnia 11 directly to a Nautel NV series exciter. Wheatstone has also unveiled its application of this technology, which is called Baseband192. Nautel and Wheatstone have both produced videos on this topic.
The operational specifics are pretty simple in concept. Think for a moment about the workings inside a typical audio processor/stereo generator combo: After all the audio processing is finished, the stereo generator, still in the digital domain, converts this audio to the familiar composite MPX format, and a final digital to analog conversion takes place. If that signal is routed via coax to the composite input of (for example) the integrated digital exciter in an transmitter, a subsequent analog to digital conversion takes place at the input of the modulator. Wouldn't it be great to eliminate that D/A and A/D conversion set? Well, by scaling the AES sample rate up to 192kHz (giving 96kHz of audio bandwidth) the intermediate steps along the way can be eliminated. The coax is replaced by an XLR cable to transport the AES data stream.
Orban's version follows the system developed by Nautel and Omnia, but samples at 384kHz and uses both left and right channels in the AES3 stream to multiplex the samples in an even-odd sequence between the left and right channels of a 192kHz AES3 link. Using only the left channel it is compatible with the Omnia method. The hardware can be retrofitted into an Optimod-FM 8600 on a rear-panel chassis cut-out.
Broadcast Electronics and Harris Broadcast offer a similar feature on at least one exciter in each of their respective lines. The feature is generically known as MPX over AES in these cases.
- continued on page 2
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