The patent landscape¶

AMBE's reputation in amateur radio is inseparable from its patents. This page tries to replace the folklore with the record: which patents cover what, when each one expired or expires, which two are still in force today, and what that actually constrains for someone building an open implementation.

Still in force¶

Two patents are live today. Both are assigned to Digital Voice Systems, Inc.

US 8,359,197 — "Half-rate vocoder" · expires 2028-05-20¶

patents.google.com/patent/US8359197B2 · inventor John C. Hardwick · filed 2003-04-01 · granted 2013-01-22 · status shown: "Active, expires 2028-05-20"

The application was filed on 1 April 2003, so a plain twenty-year term would have ended on 1 April 2023. It did not issue until 22 January 2013 — nearly ten years in prosecution — and the expiry Google Patents displays, 2028-05-20, sits about five years past the plain term. That gap is patent term adjustment for USPTO delay.

The independent claims cover both encoding and decoding. Encoding includes dividing speech into frames, computing MBE model parameters, quantizing pitch, voicing and gain, combining selected bits into a 12-bit parameter codeword, and applying error control codes. There are separate decoding claims: extracting FEC codewords from a received frame, error-control decoding them, reconstructing pitch, voicing and gain, and synthesizing speech samples. A decoder-only implementation is not outside this patent the way it would be outside an analysis-only patent.

The claims describe a 3600 bps frame: 72 bits per 20 ms, of which 49 are voice or tone bits (7 pitch, 5 voicing, 37 spectral magnitude) and 23 are FEC, using one [24,12] extended Golay code and one [23,12] Golay code to protect the 24 most error-sensitive bits and leaving the remaining 25 unprotected. The field widths are set out in the patent itself.

That is structurally the half-rate frame used by DMR, dPMR, NXDN and YSF's narrow mode. The equivalence is community consensus, not a legal finding. DVSI has never publicly equated its "AMBE+2" branding to the P25 half-rate specification, and no claim-by-claim mapping of the deployed formats onto these claims has been published.

US 8,036,886 — "Estimation of pulsed speech model parameters" · expires 2029-10-02¶

patents.google.com/patent/US8036886B2 · inventor Daniel W. Griffin · filed 2006-12-22 · granted 2011-10-11 · status shown: "Active, expires 2029-10-02"

This one runs longer but reaches far less far.

Claim 1 is a method of analysis. It divides a digitized signal into frequency bands by bandpass filtering, performs "an operation to emphasize pulse positions on at least two frequency band signals to produce modified frequency band signals," and determines pulsed parameters from those. That is an encoder-side measurement technique for the pulsed excitation class introduced by US 6,912,495. It says nothing about reconstructing speech.

Two practical consequences. A decoder does not perform this analysis and is unaffected by these claims. An encoder that needs to detect pulsed frames either has to wait, or has to arrive at that decision by a route the claims do not describe. The plain twenty-year term from the 2006 filing would have ended in December 2026; the 2029 date again reflects term adjustment for examination delay.

How to read a US patent expiry¶

Three rules cover everything below.

Applications filed before 8 June 1995 get the longer of seventeen years from grant or twenty years from filing. This is why several 1990s DVSI patents ran well past their twenty-year filing anniversary — a patent granted in 1998 could run to 2015 on the seventeen-year rule.

Applications filed after that date get twenty years from the earliest US non-provisional priority date. A continuation inherits its parent's priority date, so continuations expire with the family, not twenty years after their own filing. That is why several of the 2011–2013 grants below were already close to death when they issued.

Patent Term Adjustment (PTA) extends the term when the USPTO itself caused the delay. This is the mechanism that matters most here: it is why one patent filed in 2003 is still in force in 2026.

Two other things end a patent early. Maintenance fees are due at 3½, 7½ and 11½ years. Missing one results in the patent expiring. Google Patents shows this as Expired - Fee Related. A terminal disclaimer filed to overcome a double-patenting rejection caps a patent's term at its relative's. None of the core patents below carries one.

The expired core: full-rate MBE and AMBE¶

These are the patents that teach the codec described on the rest of this site. All of them are assigned to Digital Voice Systems, Inc. All of them have expired at the end of their natural term.

Read together they cover, in public, an entire full-rate MBE codec:

• pitch and voicing — nonlinear band pre-processing to recover a weak or missing fundamental, a hybrid frequency- and time-domain estimator, pitch-multiple search against octave errors, and voicing smoothing across time and frequency US 5,715,365; US 5,826,222; US 5,216,747;
• spectral amplitudes — computed identically for voiced and unvoiced harmonics so a band flipping voicing does not step in level, complete with a tabulated analysis window US 5,754,974;
• quantization — predict this frame's log-amplitudes from the last frame's, interpolate across a changing harmonic count, code the residual through a block DCT, and decay the predictor so channel errors die out US 5,630,011;
• the channel — bits ranked by error sensitivity, Golay and Hamming codes allocated accordingly, interleaving against bursts, pseudo-random modulation that lets the decoder detect uncorrectable errors for free, and frame repeat or mute when it does US 5,870,405;
• synthesis — an oscillator bank for voiced harmonics, shaped noise for unvoiced ones, and voiced phases regenerated from the magnitude envelope rather than transmitted US 5,701,390; US 5,081,681.

The oldest of these lapsed in 2010 and the last in May 2015. Everything this site teaches comes from this group.

The middle generation: all expired¶

The next wave covers the techniques that separate AMBE and AMBE+2 from 1990s IMBE, in particular it covers the multi-subframe quantization, the three-state excitation model, transcoding, and the enhanced full-rate vocoder. Every one of them is now expired, two of them for unpaid maintenance fees rather than by running their term.

Three of these are worth naming individually.

US 6,161,089 and US 6,199,037 are the architectural core of multi-subframe parameter coding: jointly quantizing spectral magnitudes across two consecutive subframes, and jointly quantizing the pitch and voicing of those subframes into a single codeword. That pairing is what makes a half-rate frame possible at all. Both expired in 2017.

US 6,912,495 introduced the three-state excitation model: voiced, unvoiced, and pulsed. Each has its own strength parameter varying over time and frequency extending MBE past the two-state voicing decision described elsewhere on this site. It expired on 2023-11-28. The model is public; note the caveat in the next section about one particular way of estimating its pulsed parameters.

US 8,595,002 is a continuation out of the same 2003 application family as the half-rate patent below. It inherited the 2003 priority date, so its twenty-year term ended 2023-04-01, and Google Patents shows it as fee-related expired. It does not extend the family's reach.

A third-party invalidity theory, recorded and not relied on¶

For completeness, because it circulates in the amateur community and readers will meet it.

Bruce Perens (K6BP), whose 2014 "AMBE Exposed" talk is the route by which most hams first encountered this subject, published a post-talk correction arguing that certain DVSI patents are invalid under the pre-AIA public-use bar of 35 U.S.C. § 102(b) — the theory being that AMBE was sold or used commercially more than a year before the corresponding applications were filed. He cites MPEP 2133 together with In re Blaisdell, Hall v. Macneale, and Ex parte Kuklo.

This is an unlitigated third-party theory. It is not an expiry, not a finding, and not a basis for anyone to act. No court or PTAB proceeding has tested it. The underlying commercial-use dates are not verified here. And the prediction accompanying it in 2014 — that the patents would "expire next year" — did not come to pass: US 8,359,197 has maintained its fees and its status line still reads Active in 2026.

Recording it changes nothing about the section above. Treat live patents as live.

Trademarks and independence¶

AMBE, AMBE+, and AMBE+2 are trademarks of Digital Voice Systems, Inc., used on this site only to identify the technology under discussion. This project is independent of DVSI and is not affiliated with, sponsored by, endorsed by, or approved by them.

Next: Sources and method. Every source behind this site, what was deliberately excluded, and how to check any of it. Previously: Listen: real hardware examples.

Last reviewed: 2026-08-10.