AM 620 KMKI Spectrum
- Antenna: A wide-band receive-only 'active' antenna.
- Spectrum analyzer plug in: HP 8557A
- Vertical scale: 10 dB/division
- Horizontal, center frequency: Approximately 620 KHz
- Horizontal, frequency span: Approximately 570 KHz to 670 KHz (this varies slightly on each photo as indicated)
5-02-2002 Spectrum
This spectrum photo was taken May 2nd, 2002.
The horizontal scale is 10 KHz/division.
This is a photo of a relatively clean spectrum. The wider base around the KMKI 620 is a result of program material, probably music. KSKY 660 likewise shows sidebands also due to normal program material.
40 dB down BW is approximately 20 KHz.
4-19-2005 Anomolous Spectrum
This spectrum photo was taken April 19th, 2005. The Video Filter was used, but only mildy (it is a variable control on the HP 8557A PI).
The horizontal scale is 11 KHz/division (on account of an the spectrum analyer plug-in requiring calibration).
'Anomolous' sideband are observed ever-present regardless of the program material playing on the station; a slight 'digital' noise can also be heard when tuning across AM 620's channel assignment using an analog tuning receiver.
40 dB down BW is approximately 28 KHz (compare this with approximately 10 KHz for the photo above).
When 'tuning' these sidebands, a white noise 'roar' can be heard in a commercial quality AM receiver of good design, including an AM/FM OEM car radio (as usually installed in full-size '94 Chevrolet Caprice).
Ojectionable artifacts are present (apparently) from KMKI AM 620 KHz when listening to KTBB AM 600 KHz. These artifacts are not normaly present on KTBB 600 (from any terrestrial source) as previous listening has observed.
DF bearings of the 'noise' (approximately 610 and 630 KHz) were taken with a receiver containing a directional ferrite rod antenna; all DF bearings indicate the same bearing as the carrier/normal program material on 620 KHz. This process was repeated again a couple if blocks further away from the first point (just to eliminate any possible local sources of the 'noise') and identical results were had (noise and carrier both null on same bearing).
No video Filter; this gives a better idea of the 'transient', instantaneous nature of the noise.
Max Hold on Video peak
This is a composite run of multiple spectrum analyzer 'sweeps' with the maximum value held in memory and displayed.
40 dB down BW looks to be around 30 KHz (compare this with approximately 20 KHz for the clean spectrum photo above).
Spectral 'plan' for iBiquity's AM IBOC Digital in Hybrid Mode: "iBiquity HD Radio Spectrum".
"The spectrum of the HD-Radio hybrid mode has analog audio from base band to +/- 5 kHz (or 8 kHz), the main digital carriers out to 15 kHz." (Graphic and text extracted from Reference #5.)
Actual 'test' spectra of a transmitter outputting iBiquity's AM IBOC Digital in Hybrid Mode. (Graphic extracted from Reference #5.)
Conclusion
The 'new' spectra of KMKI AM 620 uncannily resembles that of iBiquity's AM IBOC Digital in Hybrid Mode. I therefore conclude that KMKI AM 620 several weeks before 4-19-2005 went 'digital' per my observations of 'IBOC hash' in "usual proximity" to the station's main AM carrier.
References:
- FCC AM Broadcast Radio Station main page - a broad compilation of many technical documents
AM stands for "Amplitude Modulation", which refers to the means of encoding the audio signal on the carrier frequency. In many countries, AM radio stations are known as "mediumwave" (MW) stations. AM stations are also sometimes referred to as "standard broadcast stations", since AM was the was the first form used to transmit broadcast radio signals to the public.
- FCC AM Radio Database Query
Search for radio station information in the AM broadcast band (530 kHz to 1700 kHz) using a variety of criteria (callsign, City, state, by frequency or x kilometers from a given Lat/Lon coordinate).
- FCC Digital Station List
FCC: "For a list of stations authorized to transmitting both analog and digital signals, use the following links instead of the AM Query: AM Digital Stations"
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Radio Station KTBB AM 600 KHz, east Texas, center frequency 20 KHz removed from center freq assignment of KMKI AM 620 KHz.
The daytime coverage and nighttime coverage areas are available at this link; the 'distant' coverage contour (area) encompasses both my home location and the transmit site of KMKI 620 KHz.
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Linearity Performance
of AM Transmitters for
HD-Radio and DRM Performance
By Phil Schmitt and Ky Luu, Harris Broadcast Communications
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iBiquity IBOC Digital systems, on-air status, Texas
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IBOC AM Digital Radio System
Excerpt a couple of relevant terms
Glossary of Terms
The following terms are used to describe the AM IBOC system.
hybrid IBOC -
The initial mode of the iBiquity AM IBOC system that adds digital audio capacity to an AM signal by inserting digital sidebands in the spectrum above, below, and within the analog AM signal. The digital audio data rate can range from 36 kbits/s to 56 kbits/s, and the corresponding ancillary data rate is 0.4 kbits/s in both cases.
IBOC -
An in-band/on-channel system of digital radio where the digital signals are placed within the current AM and FM bands and within the FCC-assigned channel of a radio station.
...
iBiquity AM IBOC System
The iBiquity AM IBOC system supports transmission of digital audio and auxiliary digital data within an existing AM channel allocation by placing six groups of digitally modulated carrier signals within and adjacent to an analog AM signal (Figure 2.4.1) [1]. Because digitally modulated carriers are inserted within the same spectrum occupied by the analog AM signal, the AM IBOC system is not compatible with analog AM stereo signals. Corresponding sideband groups on either side of the carrier (i.e., upper primary and lower primary) are independent in that only one of them is needed for an IBOC receiver to be able to generate digital audio. However, in order to generate stereo (or enhanced fidelity) digital audio, the secondary and tertiary sideband groups are needed.
Orthogonal frequency division multiplexing (OFDM) modulation is utilized in the AM IBOC system. The digital audio modulated onto these OFDM carriers is perceptually coded, allowing for high-quality digital audio using a relatively low bit rate. At the transmission site, the audio coder creates two audio streams—a “core” stream and an “enhanced” stream—and the system assigns the streams to different parts of the spectrum. The core stream carries monaural audio and the enhanced stream, at the broadcaster’s option, carries enhanced fidelity stereo audio.
The audio codec can be set to provide stereo audio in the enhanced portion of the audio stream. At the receive site, then, the listener will hear either enhanced or core digital audio or analog audio depending upon the reception conditions. The audio bandwidth for the digital audio (both enhanced and core modes) is approximately 15 kHz.
The AM IBOC signal incorporates a 4-1/2 s delay between the analog and digital (simulcast) audio signals to improve performance in the presence of certain types of interference, which may affect how broadcasters monitor off-air signals.