THE DISCUSSION of accurate performance practice for Bach's music has intensified steadily over the past few years. Various organizations lay claims to all manner of historically-precise techniques. The Munich Bach Choir lies somewhere in the center of the spectrum, between the advocates of absolute musical historicity and the traditionalists. The Choir's performance of the B-Minor Mass makes a powerful argument for just the vital elements of Bach's composition with a firm command of standard nineteenth-century technique.

Sheer size is a clear departure from the actual forces available in Bach's time for an indoor performance. Richter's ensemble numbers 140 singles and orchestral personnel; Bach's optimum number was less than half that. The Bach Choir's is a modern one, a product of the great symphonies after 1800. Between Bach's time and our own, instruments were generally engineered to be louder and more precise in pitch. Those listening to a cantata in the 1730's were not steeped in a tradition of massive sound; the scale of volume-production then was a fraction of what it is now. The result is a real-scale performance of old music that may sound hopelessly small to many listeners. The Richter approach presents Bach in terms agreeable to a majority of people.

Yet the Mass is no Mendelssohnian adaptation from the original. Careful attention was paid to ornaments, most noticeable in the brilliantly-played trumpet parts. Richter varied his tempi quite a bit though they were generally fast by standards of only twenty years ago (such as on the Scherchen recording of 1952). Richter had the courage to vary the tempi quite a bit to his taste. The bass aria Quoniam tu solus was taken very slowly. The horn playing in the obbligato solo to this aria is the best that might ever be expected. The soloist exhibited a precision of control that allowed for the most subtle variation in tone, dynamics and attack while preserving melodic integrity. Even his placement on the stage was perfectly coordinated to bounce the horn's sound off the left front well of the hall towards the audience. The solo singer--who had an unfortunately harsh voice--was quite overshadowed by his accompaniment.

The bulk of praise for the performance is due the chorus. The singers are extraordinary. They are capable of great projection, as in the also entrance of the first Kyrie eleison; and they are equally able to sing softly and expressively, as in the Confiteor unum baptisms. Their diction was clear; and strong emphasis was reserved for truly significant moments like the Crucifixus. The Choir showed immense poles in handling violent transitions: the Et resurrexit was a shock to the audience. Only once in the Cum Sancto spiritu was an entrance made without seemingly total assurance.

In balance, too, the Choir excelled. Rather than a collection of sopranos with others, it was a true equality of vocal parts so necessary to Bach's complex polyphony. Richter generally chose legato articulation, but the choir handled the staccato Pleni sunt coeli passage with case. The attention paid their conductor is yet another point: Richter varied his tempi suddenly, but singers and orchestra followed right along.

The orchestral playing was in every way equal to the singing. In addition to the perfect horn obbligato, the other soloists were very good. The oboes, flutes, and violins all accompanied with great care. The rarely-played Aeolian-Skinner organ of Symphony Hall was used with registrations quite appropriate to its role as a continuo instrument. It was especially effective in the Cum sancto spiritu and the exultant Et expecto resurrectionem.

Richter is not an exciting conductor to watch, but he does communicate his enthusiasm to players and audience. From the very start it was clear that he was not afraid to build long phrases. The last chorus, Dona nobis pacem, was begun slowly but picked up speed in a controlled manner leading to a majestic conclusion.

The attention paid to details might well be a lesson for American ensembles. As soon as Richter turned from the audience to the performers, the music began--no tuning was done with the conductor on stage. The chorus stood up and sat noiselessly with no bother about scores or seats. Small matters, no doubt, but these were further aspects of the group's proud professionalism.

The Bach Choir presented an idealized--doubters might say distorted or impossible--image of the Mass. Far larger than the composer himself could have imagined this was not a purist's performance. It was honest enough to appreciate its own potential: valveless trumpets, natural horns, and wooden flutes all would have been senseless against a modern string section and mixed (rather than all-male) chorus. An unhappy marriage of old with new produces a far less satisfying result than either extreme followed completely.

A successful wedding of old and new techniques can mean the stunning musical experience of last Monday: an evening when the flaws were so minor as to be buried under the brilliance of performance.