A computer-based automatic speech recognition system , wherein the system uses a processor coupled with stored instructions implementing modules of the system , wherein the processor executing the instructions is configured to encode an incoming stream of acoustic frames representing features of a speech utterance into a sequence of encoder states of encoded acoustic feature frames; and successively process each encoder state of encoded acoustic feature frames with a frame-synchronous decoder, FSD, module until a termination condition is met, wherein the FSD module , in response to identifying an encoder state carrying information about a new transcription output , expands a current list of FSD prefixes to produce a candidate list of FSD prefixes and corresponding FSD scores, wherein each FSD prefix in the candidate list is an estimation by the FSD module of decoded transcription outputs in the encoder states processed by the FSD module , wherein a probability of an FSD prefix being the decoded transcription outputs is defined by the corresponding FSD score, wherein, in response to producing the candidate list of FSD prefixes , the processor is configured to select a portion of the encoder states including the encoder state identified by the FSD module and trigger a label-synchronous decoder, LSD, module to process the selected portion of encoder states to determine LSD scores defining probabilities of the FSD prefixes in the candidate list of FSD prefixes being the decoded transcription outputs in the selected portion of encoder states according to the LSD module ; prune the candidate list of FSD prefixes according to joint scores defined by a combination of corresponding FSD scores and LSD scores; and replace the current list of FSD prefixes with the pruned list of FSD prefixes to continue decoding for a next transcription output . encode an incoming stream of acoustic frames representing features of a speech utterance into a sequence of encoder states of encoded acoustic feature frames; and successively process each encoder state of encoded acoustic feature frames with a frame-synchronous decoder, FSD, module until a termination condition is met, wherein the FSD module , in response to identifying an encoder state carrying information about a new transcription output , expands a current list of FSD prefixes to produce a candidate list of FSD prefixes and corresponding FSD scores, wherein each FSD prefix in the candidate list is an estimation by the FSD module of decoded transcription outputs in the encoder states processed by the FSD module , wherein a probability of an FSD prefix being the decoded transcription outputs is defined by the corresponding FSD score, wherein, in response to producing the candidate list of FSD prefixes , the processor is configured to select a portion of the encoder states including the encoder state identified by the FSD module and trigger a label-synchronous decoder, LSD, module to process the selected portion of encoder states to determine LSD scores defining probabilities of the FSD prefixes in the candidate list of FSD prefixes being the decoded transcription outputs in the selected portion of encoder states according to the LSD module ; prune the candidate list of FSD prefixes according to joint scores defined by a combination of corresponding FSD scores and LSD scores; and replace the current list of FSD prefixes with the pruned list of FSD prefixes to continue decoding for a next transcription output . <EOS>