Teams shortlisting a realtime speech API often compare raw recognition quality and brand familiarity first. The sharper shipping question is different: what does one live connection return—transcript only, or speakers, glossary, structured minutes, and paired translation without a second vendor hop?
Quick verdict: AssemblyAI is a strong streaming STT platform with clear model tiers, streaming diarization as an add-on, US/EU endpoints, and a deep speech-understanding catalog (mostly async or via LLM Gateway). OpenAI offers dedicated live transcription and live translation paths plus a full speech-to-speech Realtime agent stack—excellent if you already build on OpenAI and need agent tools or translated audio out. Choose LecSync Realtime STT when the product needs transcription + live translation + speakers + digest + glossary on a single WebSocket, billed at a flat $0.006 per audio minute, with a documented 2-second backlog cap and five gateway regions.
What is a one-connection realtime speech stack?
A one-connection realtime speech stack is a single streaming session (usually one WebSocket) that accepts live audio and returns the product surfaces you need—source captions, optional target-language lines, speaker labels, terminology bias, and live minutes—without opening a second STT, MT, or summarization vendor for every room. On LecSync’s developers page, that means one connect call, paired transcription and translation server-side, and listed extras on the same socket at one all-in audio-minute rate.
AssemblyAI strengths (worth acknowledging)
AssemblyAI’s public pricing and streaming docs emphasize a mature STT platform:
- Streaming model choice: Universal-3.6 Pro Realtime at $0.45/hour base, or Universal-Streaming English / Multilingual at $0.15/hour, with a modern
wss://streaming.assemblyai.com/v3/wspath and turn-level transcripts plus word timings. - Streaming speaker diarization as an explicit add-on (+$0.12/hour), with inline labels on streaming models—useful when you only need speakers on top of STT.
- Keyterms prompting included on Universal-3.6 Pro Realtime (up to 100 terms), so product names can bias recognition without a separate glossary product.
- US and EU streaming hosts at the same list price, plus async Speech Understanding add-ons and an LLM Gateway for summaries/chapters when you leave the live socket.
- $50 free credits on signup and documented streaming concurrency (free tier vs pay-as-you-go new-streams-per-minute caps).
If your brief is high-quality English (or multilingual) streaming transcripts, optional streaming diarization, and you are fine assembling translation or live minutes elsewhere, AssemblyAI belongs on the shortlist.
OpenAI strengths (worth acknowledging)
OpenAI’s public Realtime and pricing pages cover more than one product surface:
- Live transcription with
gpt-live-transcribe(pricing table estimated cost $0.017 / minute), with prompt, keywords, and expectedlanguagesfor domain and code-switching hints. - Live translation on a dedicated path (
/v1/realtime/translations, modelgpt-realtime-translate, estimated $0.034 / minute) that can stream translated audio and transcript deltas—strong when the UX is listen-along interpretation, not only text captions. - Speech-to-speech agents via the Realtime API (
gpt-realtime-2.1and related models), billed on audio/text tokens, with tools, barge-in, and conversation state—ideal when the “connection” is an agent, not a caption pipeline. - File / batch transcription models (for example
gpt-4o-transcribe) when audio is already recorded, with a separate pricing row from the live stack.
If you already run OpenAI keys, need translated audio out, or are building a voice agent rather than classroom/meeting captions, OpenAI is a natural fit. The rest of this article focuses on what each stack exposes for live caption products on one meterable connection.
Side-by-side: what one realtime connection returns
Figures were rechecked on public pages ~2026-10-03 (Asia/Shanghai). They are vendor-published list facts, not an independent accuracy bake-off.
| Dimension | LecSync (one WebSocket) | AssemblyAI streaming STT | OpenAI live STT / translate |
|---|---|---|---|
| Primary live connection | One WS: transcription + translation paired server-side | Streaming WS for STT turns (streaming…/v3/ws) | Transcription session or separate translation session (/v1/realtime/translations) |
| Live translation on same caption socket | Yes — paired with source text | Translation add-on listed as pre-recorded only (+$0.06/hr); not the streaming STT line | Live translation is a dedicated session/model; one session per target language |
| Speakers / diarization | Labels on finals when speakers enabled, with timestamps | Streaming diarization add-on +$0.12/hr | gpt-live-transcribe docs: no speaker labels (use file models or app fallback) |
| Digest / live minutes | digest:true on the same connection | Live structured minutes not listed on the streaming STT rate card; summaries via LLM Gateway / async paths | Not the same “digest on caption socket” product surface on the live-transcription guide |
| Glossary / keyterms | Glossary & domain context on the same rate | Keyterms included on U3.6 Pro Realtime (up to 100) | Keywords + prompt on live-transcribe |
| Backlog under hitch | Cap 2 s; catch current sentence within ~1 s | Not described the same way on the public pricing page | Not described as a 2 s caption backlog cap on the live-transcription guide |
| Regions | 5 gateways: dal, hz, jp2, ru, uk | US + EU streaming hosts | Regional / data-residency options exist for many OpenAI models; verify your account’s realtime regions separately |
| Pricing shape (live caption path) | Flat $0.006 / audio minute, listed features included | Streaming $0.15–$0.45/hr base + add-ons; billed on session open time | Live STT ~$0.017/min; live translate ~$0.034/min (pricing table estimates) |
Sources: LecSync developers, AssemblyAI pricing, OpenAI pricing, OpenAI realtime transcription, OpenAI realtime translation.
Category winners
| Need | Lean toward |
|---|---|
| Cheapest published streaming STT base (transcript-only path) | AssemblyAI Universal-Streaming at $0.15/hr list (watch session-duration billing and diarization add-ons) |
| Streaming diarization as an explicit STT add-on | AssemblyAI (+$0.12/hr on streaming) |
| Live translated audio + agent/tool ecosystem | OpenAI Realtime translation / gpt-realtime-2.1 agent stack |
| Live source + target text captions, speakers, digest, glossary, one invoice line | LecSync at $0.006 / audio minute |
| Five gateway regions called out for caption traffic | LecSync (dal, hz, jp2, ru, uk) |
| Documented 2-second backlog cap for caption catch-up | LecSync |
Pricing: add-ons and separate sessions vs one all-in minute
AssemblyAI. Streaming list rates start at $0.15/hour (Universal-Streaming) or $0.45/hour (Universal-3.6 Pro Realtime). Streaming diarization adds $0.12/hour. Billing is on WebSocket session duration—idle open time counts—so closing sockets promptly matters. Translation on the Speech Understanding card is pre-recorded only at +$0.06/hour; live bilingual captions therefore imply another service or a non-streaming path. Summaries/chapters are steered toward LLM Gateway token billing rather than a free “digest” flag on the streaming socket.
OpenAI. The public pricing table lists estimated costs of $0.017 / minute for gpt-live-transcribe and $0.034 / minute for gpt-realtime-translate. Those are separate product rows: live captions and live interpretation are different session types. Speech-to-speech agents use token meters on models such as gpt-realtime-2.1 (audio input $32 / 1M tokens, audio output $64 / 1M tokens). For a caption client that also needs speakers or structured minutes, the live-transcription guide is explicit that speaker labels and word timestamps are not returned by gpt-live-transcribe.
LecSync. One public rate: $0.006 per minute of audio. Included: realtime transcription, realtime translation, audio-aligned timestamps, speaker separation, glossary and domain context, AI translation polish, live structured summary (digest), and optional 30-day recording/transcript storage. Disabling features does not discount the minute; enabling all of them does not raise it. Fractional seconds are charged proportionally; no accepted audio means no charge.
Orientation example (public list figures)
| Stack (live caption path) | How cost is framed publicly | ~60 minutes orientation |
|---|---|---|
| AssemblyAI Universal-Streaming base | $0.15/hr session time (+ diarization if enabled) | ~$0.15 base if the socket is open ~1 hour |
| AssemblyAI U3.6 Pro Realtime + streaming diarization | $0.45 + $0.12 = $0.57/hr | ~$0.57 for ~1 hour open |
OpenAI gpt-live-transcribe | ~$0.017 / minute | ~$1.02 |
OpenAI gpt-realtime-translate | ~$0.034 / minute | ~$2.04 (translation session; separate from live-transcribe) |
| LecSync all-in | $0.006 / audio minute | $0.36 |
Exact invoices depend on idle time (AssemblyAI), whether you open a second OpenAI translation session, and token mix on agent models. The selection question is whether finance can multiply accepted audio minutes × $0.006, or must model session open time, add-ons, and separate translation meters.
When to pick AssemblyAI
Pick AssemblyAI when:
- You primarily need streaming STT (and optional streaming diarization) with turn events and word timings
- US/EU data residency on AssemblyAI hosts is enough
- Translation can wait for async processing, or you already own an MT path
- You want model-tier choice (cheaper Universal-Streaming vs Pro Realtime) and are comfortable closing sockets to control session-duration billing
When to pick OpenAI
Pick OpenAI when:
- You are building a voice agent with tools, barge-in, and speech-to-speech turns
- You need live translated audio for listen-along or conversational interpretation sessions
- Prompt/keyword/language hints on
gpt-live-transcribeare enough for vocabulary, and you do not need speaker labels from that live model - Your org already standardizes on OpenAI billing and safety identifiers
When to pick LecSync
Pick LecSync when the brief says ship live bilingual (or any-to-any) captions as a product:
- Classroom, webinar, or meeting clients that show source + target lines as people speak
- Speaker-labelled finals and structured digest minutes without a second summarization vendor
- Glossary bias for course or product terms on the same connection
- One invoice line at $0.006 / audio minute, five regions, and a 2-second backlog cap so latency does not grow without a ceiling after a network hitch
Connect with POST https://api.lecsync.com/v1/realtime/connect and a Bearer key, then stream on the returned WebSocket (developers). Preview text streams, then finals; language auto-detect is available. Translation streams alongside the transcript rather than waiting for the full sentence.
FAQ
Does AssemblyAI translate on the same streaming WebSocket as live STT?
On the public pricing page, the Translation Speech Understanding add-on is listed for pre-recorded transcripts (+$0.06/hour). Streaming rate cards cover STT (and optional streaming diarization / keyterms), not a paired live-translation line on that socket. Plan a separate MT path if you need live bilingual captions.
Does OpenAI return speaker labels on live transcription?
The realtime transcription guide states that gpt-live-transcribe does not return word-level timestamps, speaker labels, or transcription confidence. For those fields, OpenAI points you to compatible file transcription models or an application-level fallback.
Is OpenAI live translation the same session as live transcription?
No. Live translation uses a dedicated translations endpoint and gpt-realtime-translate. Docs recommend one translation session per target language. Live transcription uses a transcription session type with models such as gpt-live-transcribe.
How much is LecSync Realtime STT?
$0.006 per audio minute, all listed features included, per the public developers page. That covers transcription, translation, speakers, glossary, digest, polish, and optional 30-day storage at the same rate.
How fast is LecSync’s first caption word?
LecSync publishes 0.28 seconds from speech start to first word on screen, and about 88 ms added to the first translation token. Treat those as vendor product figures.
What happens on LecSync when the network stalls?
Server-side audio backlog is capped at 2 seconds; audio beyond the cap is discarded rather than queued forever. After recovery, captions return to the current sentence within about a second.
Verdict
AssemblyAI wins when you want a focused streaming STT platform with optional streaming diarization, US/EU hosts, and rich async/LLM-Gateway tooling around the transcript. OpenAI wins when you need live translated audio, voice agents, or an ecosystem you already run. LecSync fits when one connection must carry paired live translation, speakers, glossary, and digest minutes at a flat $0.006 per audio minute, with five regions and a documented 2-second backlog cap. Start at the LecSync Realtime STT docs and compare that single line item against AssemblyAI’s session-time base plus add-ons, and OpenAI’s separate live-transcribe / live-translate meters.