WCCCA/Motorola SmartZone Analog Technical "hum" question

[V_A_R_I_A_B_L_E]

Always wondered but never got around to asking until now....does anyone know what that low-frequency audible "hum" sound is after every transmission on the WCCCA system? (I think the analog 800Mhz Portland system used to do it to?) If someone with technical knowledge of how these analog trunking systems work happens to know the technical answer, I'm quite curious. You can hear it in this recording from the attempted helicopter hijacking at KHIO a few years ago.

 

[radiotweester]

I think what you're refering to is the access tone? Just like conventional, a sub-audiable tone is sent to access the repeater the control channel has directed the radio to for their TG. All Analog TRS have 1 of the 8 tones. They aren't the standard CTCSS. It's sort of a security (besides system key and valid radio ID) and also acts as a noise reducer like CTCSS or DCS does on analog. BOEC had the same but different tone.
If you're recording, I'm doubting there's the 300Hz cutoff like you'd hear out of the speaker of a radio or scanner and the receiver is passing the tone. There is or was 1 of the WCCCA main repeaters on 859.xxxx had an extra ordinary amount of noise on TX almost to the point of not being understandable. I've not listened for a while, so it might have been fixed.
Being around radios for decades, I don't even hear the hum in the recording .

 

[V_A_R_I_A_B_L_E]

I think what you're refering to is the access tone? Just like conventional, a sub-audiable tone is sent to access the repeater the control channel has directed the radio to for their TG. All Analog TRS have 1 of the 8 tones. They aren't the standard CTCSS. It's sort of a security (besides system key and valid radio ID) and also acts as a noise reducer like CTCSS or DCS does on analog. BOEC had the same but different tone.
If you're recording, I'm doubting there's the 300Hz cutoff like you'd hear out of the speaker of a radio or scanner and the receiver is passing the tone. There is or was 1 of the WCCCA main repeaters on 859.xxxx had an extra ordinary amount of noise on TX almost to the point of not being understandable. I've not listened for a while, so it might have been fixed.
Being around radios for decades, I don't even hear the hum in the recording .

Sub-audible tone was my first thought, however the hum only appears briefly, not during the whole transmission....and it appears only *after* the transmission is complete and not at the beginning, which doesn't make sense to me as a form of authentication.

If you didn't notice it in the youtube video, take a look again, as they conveniently included a graphical waveform overlay that shows it visually for ya, pretty well actually..

(Also according to the database, the "connect tone" for WCCCA's 800Mhz system is 105.88hz, but the tone in the video is definitely a lower frequency, probably between 70-80hz.)

 

[mikewazowski]

Could be the end code being sent.

 

[kf8yk]

Connect tones are only used on uplink (mobile to system) transmissions. 150 baud subaudible low speed data is mixed with voice on the downlink side.

In the video you can hear the rumble of the low speed data under the voice, with the end code sent just before the repeater de-keys. Motorola radios include a 300 Hz high pass audio filter to block the low frequency rumble from the receive audio.

The end code, or 'disconnect word' in Motorola lingo is more noticeable in the audio as it's sent at 300 baud.

 

[radiotweester]

Total agreement with kf8yk.
Disconnect data (similar to a reverse burst conventionally) to silence repeater hangtime. It also makes actual radios on on that TG mute (MUCH nicer to listen to)