  We describe a system that generates speaker-annotated transcripts of meetings
by using a virtual microphone array, a set of spatially distributed
asynchronous recording devices such as laptops and mobile phones. The system is
composed of continuous audio stream alignment, blind beamforming, speech
recognition, speaker diarization using prior speaker information, and system
combination. When utilizing seven input audio streams, our system achieves a
word error rate (WER) of 22.3% and comes within 3% of the close-talking
microphone WER on the non-overlapping speech segments. The speaker-attributed
WER (SAWER) is 26.7%. The relative gains in SAWER over the single-device system
are 14.8%, 20.3%, and 22.4% for three, five, and seven microphones,
respectively. The presented system achieves a 13.6% diarization error rate when
10% of the speech duration contains more than one speaker. The contribution of
each component to the overall performance is also investigated, and we validate
the system with experiments on the NIST RT-07 conference meeting test set.
