P25 Basics (Plain English)
- P25 Conventional: One agency stays on one frequency. You can program it like a normal channel.
- P25 Trunked: One system uses many frequencies. The control channel tells the radios where to go for each conversation (talkgroup).
- System: The overall trunked network (county/state system).
- Site: A tower/cell you receive (often the most important part).
- Talkgroup (TGID): A “channel” on trunking (Police Dispatch, Fire Tac, etc.).
- ID Search vs ID Scan: Search = discover all TGs, Scan = only TGs you saved.
Step-by-Step: P25 Trunked (Most Common Public Safety)
You need: the correct site frequencies for the tower you can receive (control channel / alternate control channels). Talkgroups are optional if you start in ID Search.
- Create a Favorites List: MENU → Manage Favorites → New Favorites List.
- New system: Open that list → Review/Edit System → New System → P25 Trunk → name it.
- Add a site: Edit Site → New Site → name it (closest site/tower).
- Enter site frequencies: Edit Site → Set Frequencies → enter the listed control/voice frequencies for that site.
- Choose mode: Start with ID Search to discover active TGs. Switch to ID Scan once you’ve saved what you want.
- Add departments/talkgroups (optional): Edit Department → New Department → Edit Channel → New Channel (enter TGIDs you want for a clean setup).
- Scan and refine: Use Avoid/Hold to clean up, and confirm you’re receiving the best site.
Step-by-Step: P25 One-Frequency (Digital on a Single Channel)
You need: one frequency. This is common for smaller agencies or special-use channels.
- MENU → Manage Favorites → select list → Review/Edit System → New System → P25 One Frequency.
- Edit Department → New Department → name it.
- Edit Channel → New Channel → enter the frequency → set name.
- Scan and verify decode. If audio is broken, improve signal quality (move/antenna) or verify it isn’t encrypted.
Sentinel vs Manual: Fastest Path for P25
- Sentinel (recommended): Easiest way to pull correct systems/sites from the database, then write a clean Favorites List.
- Manual (on-radio): Best for quick edits, quick holds, and field programming when you already know the correct data.
If you’re on Windows, start with the Sentinel guide and then use this page to understand what matters for P25 (sites, control channels, and ID Search/Scan).
Troubleshooting: “Nothing to Scan”, No Audio, and Bad Decode
- Nothing to Scan: Check Service Types, Quick Keys, Avoids, and Location Control.
- Wrong site: If you programmed the wrong site, the system may show nothing or stop with no voice. Confirm the nearest/strongest site.
- Encryption: Encrypted talkgroups can’t be monitored (you may see activity with no intelligible audio).
- Simulcast distortion: Try small moves and a smaller 700/800 MHz antenna; see the simulcast guide for step-by-step fixes.
Frequently Asked Questions
Do I need an upgrade key for P25 on the SDS100?
No. APCO P25 Phase I and Phase II are included on the SDS100. Optional upgrade keys are for formats like DMR, NXDN, and ProVoice.
What is a P25 control channel and why does it matter?
On a P25 trunked system, the control channel is a continuously-transmitting data channel that tells radios (and scanners) what voice frequency to tune to for each talkgroup. If you don’t have the correct site/control channel data, trunk tracking usually won’t work.
What’s the difference between ID Scan and ID Search on the SDS100?
ID Search lets you hear/discover any active talkgroup on the system. ID Scan restricts audio to only the talkgroups you saved. Use ID Search to explore, then switch to ID Scan for a clean, focused setup.
Why do I see talkgroup activity but hear no audio on P25?
The most common cause is encryption. If a talkgroup is encrypted, the SDS100 cannot decode intelligible audio. Other causes include weak/garbled decode (simulcast), wrong site frequencies, or scanning restrictions (service types, avoids, quick keys).
How do I improve P25 simulcast decoding on the SDS100?
Start with physical changes: move the radio a few feet, try a smaller 700/800 MHz antenna, and avoid high-gain wideband antennas indoors. If needed, lock onto the best site and review safe settings like attenuation.