...weight

often referred to as a codebook exponent.

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...probability

Since the output distributions are densities, these are not really probabilities but it is a convenient fiction.

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...non-emitting

To understand equations involving a non-emitting state at time t, the time should be thought of as being 64#64 if it is an entry state, and 65#65 if it is an exit state. This becomes important when HMMs are connected together in sequence so that transitions across non-emitting states take place between frames.

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...accumulators

Note that normally the summations in the denominators of the re-estimation formulae are identical across the parameter sets of a given state and therefore only a single common storage location for the denominators is required and it need only be calculated once. However, HTK supports a generalised parameter tying mechanism which can result in the denominator summations being different. Hence, in HTK the denominator summations are always stored and calculated individually for each distinct parameter vector or matrix.

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...operation

They can even be avoided altogether by using a flat start as described in section 8.3.

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...minor

In practice, a good deal of extra work is needed to achieve efficient operation on large training databases. For example, the HEREST tool includes facilities for pruning on both the forward and backward passes and parallel operation on a network of machines.

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...Model

See ``Token Passing: a Conceptual Model for Connected Speech Recognition Systems'', SJ Young, NH Russell and JHS Thornton, CUED Technical Report F_INFENG/TR38, Cambridge University, 1989. Available by anonymous ftp from svr-ftp.eng.cam.ac.uk.

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...used

Available by anonymous ftp from svr-ftp.eng.cam.ac.uk/comp.speech/dictionaries/beep.tar.gz. Note that items beginning with unmatched quotes, found at the start of the dictionary, should be removed.

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...files

Not to be confused with files containing edit scripts

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...arguments

Most UNIX shells, especially the C shell, only allow a limited and quite small number of arguments.

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...together

Note that if the transition matrices had not been tied, the CO command would be ineffective since all models would be different by virtue of their unique transition matrices.

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...is

All of the examples in this book assume the UNIX Operating System and the C Shell but the principles apply to any OS which supports hierarchical files and command line arguments

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...element)

Block sizes typically grow as more blocks are allocated

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...retries

This does not work if input filters are used.

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...units

The somewhat bizarre choice of 100nsec units originated in Version 1 of HTK when times were represented by integers and this unit was the best compromise between precision and range. Times are now represented by doubles and hence the constraints no longer apply. However, the need for backwards compatibility means that 100nsec units have been retained. The names SOURCERATE and TARGETRATE are also non-ideal, SOURCEPERIOD and TARGETPERIOD would be better.

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...input

This method of applying a zero mean is different to HTK Version 1.5 where the mean was calculated and subtracted from the whole speech file in one operation. The configuration variable V1COMPAT can be set to revert to this older behaviour.

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...databases

Many of the more recent speech databases use compression. In these cases, the data may be regarded as being logically encoded as a sequence of 2-byte integers even if the actual storage uses a variable length encoding scheme.

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...function

Note that some textbooks define the denominator of equation 5.4 as 145#145 so that the filter coefficients are the negatives of those computed by HTK.

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...parameterisation

In any event, setting the compatibility variable V1COMPAT to true in HPARM will ensure that the calculation of energy is compatible with that computed by the Version 1 tool HCODE.

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...173#173

Unless V1COMPAT is set to true.

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...discarded

Some applications may require the 0'th order cepstral coefficient in order to recover the filterbank coefficients from the cepstral coefficients.

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...letters

Some command names have single letter  alternatives for compatibility with earlier versions of HTK.

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...234#234

No current HTK tool can estimate or use these.

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...element

Covariance matrices are actually stored internally in lower triangular form

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...<InvCovar>

The Choleski storage format is not used by default in HTK Version 2

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...matrix

Transform matrices are not used by any of the supported HTK tools.

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...definition

The fact that this is possible does not mean that it is recommended practice!

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...brackets

This definition covers the textual version only. The syntax for the binary format is identical apart from the way that the lexical items are encoded.

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...states

Integer numbers are specified as either char or short. This has no effect on text-based definitions but for binary format it indicates the underlying C type used to represent the number.

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...mapping

The physical HMM which corresponding to several logical HMMs will be arbitrarily named after one of them.

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...likely

Remember that discrete probabilities are scaled such that 32767 is equivalent to a probability of 0.000001 and 0 is equivalent to a probability of 1.0

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...HMMs

Also called semi-continuous HMMs in the the literature.

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...words

More precisely, nodes represent the ends of words and arcs represent the transitions between word ends. This distinction becomes important when describing recognition output since acoustic scores are attached to arcs not nodes.

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...definitions

Large HMM sets will often be distributed across a number of MMF files, in this case, the -H option will be repeated for each file.

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...10

The default behaviour of HRESULTS is slightly different to the widely used US NIST scoring software which uses weights of 3,3 and 4 and a slightly different alignment algorithm. Identical behaviour to NIST can be obtained by setting the -n option.

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...words

All the examples here will assume that each label corresponds to a word but in general the labels could stand for any recognition unit such as phones, syllables, etc. HRESULTS does not care what the labels mean but for human consumption, the labels SENT and WORD can be changed using the -a and -b options.

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...NAME=11415> word

The HLED EX command can be used to compute phone level transcriptions when there is only one possible phone transcription per word

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...control

The underlying signal number must be given, HTK cannot interpret the standard Unix signal names such as SIGINT

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...options

HCOPY thus takes on the functionality of the former HTK tool HCODE, which no longer exists in V2.0.

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...algorithm

This algorithm is significantly different from earlier versions of HTK where K-means clustering was used at every iteration and the Viterbi alignment was limited to states

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...ignored

Prototypes should either have GConst set (the value does not matter) to avoid HTK trying to compute it or variances should be set to a positive value such as 1.0 to ensure that GConst is computable

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...labels

In earlier versions of HTK, HLED command names consisted of a single letter. These are still supported for backwards compatibility and they are included in the command summary produced using the -Q option. However, new commands introduced in version 2.0 have two letter names.

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...TARGETKIND

The TARGETKIND is equivalent to the HCOERCE environment variable used in earlier versions of HTK

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...grammar.

The expression between double angle brackets must be a simple list of alternative node names or a variable which has such a list as its value

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...recognition

In HTK V2 it is preferable for these context-loop expansions to be done automatically via HNET, to avoid requiring a dictionary entry for every context-dependent model

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...used

If the base-names or left/right context of the context-dependent names in a context-dependent loop are variables, no $ symbols are used when writing the context-dependent nodename.

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...revised

With the added benefit of rectifying some residual bugs in the HTK V1.5 implementation

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...mechanism

Using this option only makes sense if the HMM has skip transitions

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...transcriptions

The choice of ``Sentence'' and ``Word'' here is the usual case but is otherwise arbitrary. HRESULTS just compares label sequences. The sequences could be paragraphs, sentences, phrases or words, and the labels could be phrases, words, syllables or phones, etc. Options exist to change the output designations `SENT' and `WORD' to whatever is appropriate.

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...mode

It is not,of course, necessary to have multiple processors to use this program since each `parallel' activation can be executed sequentially on a single processor

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